Phreatic eruptions and gas-and-steam plumes persisted during January-June 2023

Rincón de la Vieja is a remote volcanic complex in the Guanacaste Range in NW Costa Rica. It consists of an elongated, arcuate NW-SE-trending ridge constructed within the 15-km-wide Guachipelín caldera, whose rim is exposed on the south side. Eruptions dating back to the 16th century have been from the active crater located ENE of Von Seebach crater; the active crater also contains a 500-m-wide acid lake. The current eruption period began June 2021 and has more recently consisted of weak phreatic explosions (BGVN 48:01). This report covers activity during January through June 2023 using information from weekly bulletins and occasional daily reports from the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA).

Activity was relatively low during January through February 2023. An eruption was reported at 1500 on 7 January, followed by observations of gray water in the Niño River. According to OVSICORI-UNA, these observations may have indicated that some material from the eruption came out of the crater. During 0537-0547 on 8 January a series of eruptions was detected.

On 2 March at 0651 and on 4 March weak gas-and-steam emissions were recorded, rising less than 500 m above the crater. A series of small eruptions occurred during 2305 on 10 March to 0137 on 11 March with at least four pulses confirmed in webcam images at 2305, 0000, 0006, and 0137. Continuous tremor was reported on 11 March. Weak gas-and-steam plumes sometimes rose higher than 500 m above the crater, but no ash was visible. Two eruptions were detected at 0529 and 0549 on 14 March, which generated a gas-and-steam plume that rose 2 km above the crater and drifted W and NW (figure 38). On 19 March at 1051 small gas-and-steam emissions rose less than 500 m above the crater.

Figure 38. Photo of a white gas-and-steam plume rising 2 km above the crater rim of Rincón de la Vieja on 14 March 2023. Courtesy of OVSICORI-UNA.

Tremor was reported starting around 2230 on 10 April. A small phreatic eruption was reported on 11 April; several pulses occurred between 0653 and 0700, the largest of which was recorded at 0656. A resulting gas-and-steam plume rose 500 m above the crater. Another gas-and-steam plume was recorded at 0743; tremor decreased in amplitude after 0900. Two small eruptions were recorded during the night and early morning during 11-12 April; the strongest of which occurred at 0450 and generated a gas-and-steam plume that rose 500 m above the crater and drifted SW. Two more eruptions were detected at 1920 on 12 April and at 0632 on 13 April; gas-and-steam plumes accompanied these events, but no ash was observed. On 14 April at 0455 low-frequency earthquakes were detected and an eruption was reported at 0935 that continued until after 2100; the resulting gas-and-steam plume rose 2 km above the crater and drifted SW. Another eruption was detected at 1826 on 14 April. On 16 April a small eruption was recorded at 0647 and lasted approximately one minute and generated a small lahar in the upper part of the basin. An accompanying gas-and-steam plume rose 2 km above the summit (figure 39). Another eruptive event occurred at 0957 and produced a gas-and-steam plume that rose 500 m above the summit. On 19 April several eruptions generating gas-and-steam plumes were reported, which rose as high as 1.5 km above the crater and drifting W. On 21 April at 1550 a strong eruption was reported. The accompanying gas-and-steam plume rose 4-5 km above the crater and drifted SW (figure 40). Lahars were observed on the N flank. Continuous degassing was reported after this eruption event, as well as strong tremor. On 21 April at 1904 a small eruption was detected in infrasound and seismic records. Near-continuous degassing persisted in the crater. On 22 April at 1904 a small eruption was detected in infrasound and seismic records and near-continuous degassing was observed in the crater. Three gas-and-steam emissions were detected on 26 April at 0453, 0523, and 0545. An eruption event at 0528 on 27 April generated a gas-and-steam plume that rose 800 m above the crater. A small eruption at 1030 on 30 April produced a gas-and-steam plume that rose 500 m above the crater accompanied by a near-continuous tremor. Several volcano-tectonic (VT) earthquakes were also detected both distally and proximally to the crater.

Figure 39. Photo of an eruption at Rincón de la Vieja at 0647 on 16 April 2023 that generated a white gas-and-steam plume and a small lahar in the upper part of the basin. Courtesy of OVSICORI-UNA.

Figure 40. Photo of the strong eruption at 1550 on 21 April 2023. The accompanying gas-and-steam plume rose 4-5 km above the crater and drifted SW. Courtesy of OVSICORI-UNA.

A moderate phreatic eruption was recorded at 1306 on 1 May that generated a gas-and-steam plume that rose 1 km above the crater. Residents to the N, seismographs, infrasound systems, and webcams all recorded the event. Seismographs recorded intermittent volcanic tremor signal and several VT-type earthquakes were recorded both distally and proximally to the crater. A small phreatic eruption was recorded at 1847 on 5 May based on data from seismographs and the infrasound system. Intermittent volcanic tremor and several low-frequency volcanic earthquakes were also detected. A short-lived explosive and energetic eruption was detected at 0258 on 14 May that produced lahars that flowed down the Pénjamo River, the Azul River, and the Azufrado River. Volcanic tremor decreased substantially on 13 May, though continuous degassing was reported. On 17 May four small phreatic eruptions were recorded, one of which occurred at 1255 that generated a gas-and-steam plume that rose 700 m above the crater. Gas-and-steam emissions persisted from the Main Crater and intermittent volcanic tremor were recorded. A gas-and-steam plume was detected in webcam, infrasound, and seismic data at 1349 on 22 May that rose 1 km above the crater. A gas-and-steam plume rose 2.5 km above the crater at 1815 on 24 May possibly containing some material from the lake.

Two moderate phreatic eruptions were detected at 1815 and 1830 on 24 May that produced gas-and-steam plumes and rose 2.5 km above the crater. The eruptions were detected by residents to the N, seismographs, the infrasound system, and webcams. Intermittent volcanic tremor was also recorded. Aerial photographs taken from 24 May showed a low water level in the lake of the Main Crater and that the lake was milky gray in color. The surface around the crater was covered in recently erupted white ash deposits (figure 41). Intermittent eruptions were reported during 25-26 May at 1435, 2230, 0453, and 0704, the strongest of which occurred at 1435. Small, low-frequency volcanic earthquakes and VT-type events proximal to the crater continued, but in smaller numbers. Intermittent degassing has persisted and there was a notable decrease in the lake level. Several eruptions were detected at 1357, 2248, and 2348 on 26 May and at 0221 and 0632 on 27 May. An eruption at 2135 on 27 May ejected incandescent material above the crater and generated a significant lahar on the N flank of the volcano that descended the Pénjamo River. The resulting gas-and-steam plume exceeded the field of view of the webcam, so OVSICORI-UNA estimated that it reached at least 3.5 km above the crater, and possibly as high as 4 km above the crater. On 29 May an eruption was observed at 0244 that ejected incandescent material and generated a lahar on the N flank of the volcano that descended the Pénjamo River.

Figure 41. Aerial photo of the Main Crater of Rincón de la Vieja taken on 24 May 2023. The active crater is covered in white ash deposits from recent eruptions. Courtesy of OVSICORI-UNA.

During 1-2 June five moderate phreatic eruptions were detected, the most energetic of which occurred at 0902 on 2 June. An accompanying gas-and-steam plume rose 1.5 km above the crater (figure 42). Seismographs intermittently recorded volcanic tremors. According to the Sentinel satellite, a sulfur dioxide plume weighing 37 tons was recorded on 2 June that drifted E. On 3 June at 0624 a gas-and-steam plume rose 1.5 km above the crater and drifted W. Some low magnitude VT-type events were recorded S of the Pailas area of the Rincón de la Vieja Volcano National Park. Sulfur dioxide emissions were recorded at average values of less than 300 tons/day. On 4 June at 0526 an eruptive plume reached 500 m above the crater, accompanied by a vertical gas-and-steam plume that rose 3.5 km above the crater and drifted N. OVSICORI-UNA reported that ash remained in the crater and there were no reports of it falling outside the summit. An eruption was recorded at 1804 on 5 June generated a gas-and-steam plume that rose 1 km above the crater, based on data from webcams, seismographs, infrasound sensors, and ground observations made to the N of the volcano. On 6 June at 0259 a small phreatic eruption was recorded that produced a gas-and-steam plume 3 km above the crater and drifted NW. On 7 June at 0142 a small eruption lasted 14 minutes based on webcam images. The eruption consisted of two pulses, the second of which was slightly more energetic. One eruption was recorded at 1505 on 7 June, but due to weather conditions the height of the plume could not be observed. Another eruption at 0942 on 8 June generated a gas-and-steam plume that rose 2.5 km above the crater. A moderate phreatic eruption was recorded at 1810 on 8 June that ejected lake water with sediment to less than 100 m above the crater rim; the accompanying white gas-and-steam plume rose 3 km above the crater and drifted W. Small phreatic eruptions were recorded at 0357, 0521 and 0546 on 11 June, the latter two of which produced a white gas-and-steam plume that rose 1.5 km and 2.5 km above the crater, respectively and drifted W. On 18 June small eruptions were recorded at 0440 and 0904 and continuous degassing was observed. A small phreatic eruption at 1804 on 18 June generated a gas-and-steam plume that rose 2 km above the crater. Several small, low-energy eruptions were recorded during the night of 25-26 June, the last one of which was recorded at 0547 that generated a gas-and-steam plume that rose 1.5 km above the crater.

Figure 42. Image showing a white gas-and-steam plume rising 1.5 km above the crater of Rincón de la Vieja at 1907 on 2 June 2023. Courtesy of Carol Quesada via OVSICORI-UNA.

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/, https://www.facebook.com/OVSICORI/).

Information is preliminary and subject to change. All times are local (unless otherwise noted)

May 1969 (CSLP 41-69)

Ash eruption on 22 April

Card 0521 (01 May 1969) Ash eruption on 22 April

"Initial activity appears to have occurred on 22 April at 1630 with ash eruption from 8,000 to 10,000 feet. No further ash eruptions reported. Observer also reports number of 'gas eruptions like funnel of ship' with increasing activity from 22 to 30 April with strong gas eruptions from 1300 to 1400 on 29 April. While volcanic activity at Rincón de la Vieja occurs relatively frequently, local opinion characterizes present eruptions as somewhat unusual."

Card 0531 (06 May 1969) Fumarole erupting dense gas

"Observation top crater Rincón de la Vieja obscured by heavy cloud cover, but observer reports a fumarole located at Quebrada Grande (same volcano) was erupting very strong and dense sulfuric gases. Observer reports this as being 'an extraordinary phenomenon.'"

Information Contacts:
Card 0521 (01 May 1969) Richard Berg, American Embassy, San José, Costa Rica.
Card 0531 (06 May 1969) Richard Berg, American Embassy, San José, Costa Rica.

June 1969 (CSLP 41-69)

No signs of activity observed

Card 0568 (04 June 1969) No signs of activity observed

The following is from a cable received on 3 June. On or about 21 May Rodrigo Sáenz observed the main crater and reports no sign of activity.

Information Contacts: Richard Berg, American Embassy, San José, Costa Rica

September 1969 (CSLP 41-69)

Ashfalls on 20 September

Card 0763 (24 September 1969) Ashfalls on 20 September

The following cable was received on 23 September. "Volcanic ash fell on Liberia and surrounding areas during early a.m., early p.m., and late p.m. on 20 September 1969 reportedly from eruption of Rincón de la Vieja. Because of heavy rains and cloud cover on 21 and 22 September impossible to report any further details whether activity is continuing at this time."

Information Contacts: Richard Berg, American Embassy, San José, Costa Rica.

August 1970 (CSLP 73-70)

New eruption on 25 August produces great ash clouds

Card 0996 (26 August 1970) New eruption on 25 August produces great ash clouds

The following cable was received on 25 August 1970. "A new eruption of the Rincón de la Vieja volcano was reported at 1100 GMT, 25 August with great clouds of ashes and smoke. The first eruption of the volcano was reported at 0549 local time 25 August. Dispatches received from Liberia reported that the volcanic activity is similar to that of the last eruption of the Irazu volcano."

Card 0999 (27 August 1970) Most ash blown over the ocean

The following cable was received on 27 August 1970 from the American Embassy. "Rodrigo Sáenz reports single eruption at Rincón de la Vieja at 0500 on 25 August produced minor ash cloud. Some ashfall over Liberia but most apparently blown to sea by winds. Ash clouds height estimated 1,000 feet. Sáenz states eruption similar to one he witnessed 14 August. No eruptions occurred between 14 and 25 August."

Information Contacts:
Card 0996 (26 August 1970) Rodrigo Sáenz R., Seccion Sismologica y Vulcanologia de Industria y Comercio, San José, Costa Rica (via San José Radio).
Card 0999 (27 August 1970) Rodrigo Sáenz R., Seccion Sismologica y Vulcanologia de Industria y Comercio, San José, Costa Rica.T. Solitario, American Embassy, San José, Costa Rica.

November 1982 (SEAN 07:11) Cite this Report

No activity at the summit crater

"In November the odor of H₂S was present from the summit crater, but there was no visible gas plume and no eruptive activity. Low-temperature steam vents and mudpots persisted at Estación las Pailas, at the foot of the volcano."

Information Contacts: R. Stoiber, S. Williams, H.R. Naslund, C. Connor, J. Prosser, and J.B. Gemmell, Dartmouth College; E. Malavassi and J. Barquero H., Univ. Nacional, Heredia.

March 1983 (SEAN 08:03) Cite this Report

Tephra eruption from crater lake

This paragraph is primarily from a report by Jorge Barquero H. and Juan de Dios Segura. During the night of 6 February, residents of towns (Dos Ríos de Upala, Colonia Blanca, and Colonia Libertad) 8 km N and NE of the volcano heard strong rumblings and observed the rise of a large eruption column from the crater. Personnel from the Proyecto de Investigaciones Vulcanológicas climbed the volcano 19 February. The odor of sulfur was stronger than it had been during their previous ascent in November 1982. Phreatomagmatic eruptions had ejected bombs, lapilli, and ash, as well as blocks 10-100 cm in diameter that formed impact craters. Tephra fell SE, S, and SW of the vent to a distance of about 1.5 km. Destruction, primarily to vegetation, was greatest to the SE and S. The tephra had a high water content because the vent contained a lake. Strong rains and rapid erosion since the eruption made it difficult to calculate the original depth of the airfall deposits, although in some places SE of the vent they were 4 cm thick. The eroded ash washed into a ravine, producing a small mudflow in a NE flank river (Río Pénjamo), causing the deaths of thousands of fish 7-8 February, possibly because of the acidity of the water. The pH of the cold lake was 3.5 on 19 February and 4.1 on 5 March.

Jorge Barquero H., J. Bruce Gemmell, and Jerry Prosser climbed the volcano on November 1982, and Gemmell provided the following report. "Rincón de la Vieja is a large composite volcano with a series of collapse craters aligned ENE-WSW. Its main cone is covered with thick vegetation but three craters to the W are not vegetated. The most recently active crater (250 m in diameter) is 1 km NW of the main cone. No activity or gas emissions were seen in this crater and a cold yellowish-green lake covered the crater floor. No steam was rising from the lake but two areas of brown discoloration near its center may have indicated subaqueous vents. The area around the summit craters was covered with accessory blocks of andesitic lava and tuff breccias, in addition to juvenile andesitic breadcrust bombs, lapilli, and ash from the most recent recorded eruptions in 1966-70. Numerous mudpots, hot springs, and steam vents occurred in two main areas (Aguas Termales and Sitio Hornillas), on the S flank at about 900 m elevation."

Further Reference. Barquero, J., and de Diós Segura, J., 1983, La Actividad del Volcán Rincón de la Vieja: Boletín de Vulcanología, no. 13, p. 5-10.

Information Contacts: J. Barquero H. and J. de Dios Segura, Univ. Nacional, Heredia; J.B. Gemmell and J. Prosser, Dartmouth College; R. Sáenz R., Ministro de Energía y Minas; La República, San José.

April 1986 (SEAN 11:04) Cite this Report

Tephra and trees down from fall 1985 eruption

The following reports are from the Observatorio Vulcanológico y Sismologico de Costa Rica (OVSICORI). "After several attempts to climb to the summit of Rincón de la Vieja were turned back by bad weather, we reached the active crater on 19 April with W. Melson of the Smithsonian Institution. During this visit, we were able to confirm that there had been a recent eruption, since we encountered recently erupted material and a devastated area, both SE of the crater.

"At the time of our previous ascent, in August 1985, this tephra had not been deposited. In a photograph taken 25 November 1985 by E. Valverde, it is possible to observe white tephra. In addition, the seismic station at the base of the volcano registered an increase in activity (harmonic tremor and A- and B-type events) between September and November, suggesting that the eruption occurred during that time.

"A fan-shaped area of about 0.25 km³ was affected. The ejecta reached a maximum distance of 500 m SE of the active crater. The erupted material is secondary, including ash, sand, and blocks as much as 20 cm across. On one rock, an ash deposit 6 cm thick was observed. The effect of the eruption on rain forest vegetation was marked about 500 m SE of the crater (in the E bank of the Quebrada Azufrosa) where trees had been knocked down in a radial pattern by the activity. This pattern is unusual in that the fallen trees appeared to radiate from a point near their center, not from the crater. In addition, various plant species in this area were affected by the acid in the pyroclastics and the associated water.

"On 19 April there was a strong and constant emission of gas that affected breathing because of its acidity, and made it difficult to observe the lake in the active crater."

Information Contacts: J. Barquero and E. Fernández Soto, OVSICORI.

February 1987 (SEAN 12:02) Cite this Report

Small explosion

"Alfonso Bustos of Upala, a town about 35 km NE of the volcano, reported that he observed an eruption of Rincón de la Vieja during the night of 31 December 1986. The seismological station of the Observatory, located on the volcano, registered an eruptive event at 2307:13 with a duration of 6 minutes 13 seconds. The period of highest amplitude was constant at one second.

"Because of weather problems in the region, we could not climb to the summit until 7 February 1987, when we verified that a small eruption had recently occurred. The affected area was the S and SE parts of the active crater, where eruptive materials were encountered to 500 m distance. At the crater rim, 10 cm of ash was measured and there were blocks 50 cm long by 40 cm wide. No evidence of juvenile material was encountered. Some plants had burns on their leaves, possibly caused by the acidity of ash deposited on them and the acidity of water that accompanied the ejecta; the eruptions occurred in a crater with a lake. The activity during the day of the ascent was constant emission of gas with a strong sulfurous odor that irritated the eyes and annoyed everyone that we encountered near the crater."

Information Contacts: J. Barquero and E. Fernández Soto, OVSICORI.

April 1987 (SEAN 12:04) Cite this Report

Phreatic eruption; lahars

"On 1 April, an eruption of Rincón de la Vieja was heard and seen by residents of the town of Buenos Aires de Upala, 7 km NE of the crater. This phreatic eruption originated from the active crater.

"Various rivers have headwaters on the N flank and flow NE (among them the Río Azul, Quebrada Azufrada, and Río Pénjamo) and lahars formed in some of them. At about 500 m above sea level, the Río Pénjamo and the Quebrada Azufrada left their channels, flooding agricultural land. No lahar formed in the Río Azul, although on 9 April it still had a gray color because of the quantity of suspended ash that it was carrying.

"According to a resident of the area, the eruption occurred at 0940. Twenty minutes later the lahar passed near the town of Buenos Aires. The seismic station of the Observatorio Vulcanológico y Sismológico registered an event at 0932.55 with a duration of 3 minutes."

Information Contacts: J. Barquero, OVSICORI.

April 1989 (SEAN 14:04) Cite this Report

Crater lake sampled

Geologists sampled the crater lake on 6 April. The lake temperature was 45°C, determined by throwing a bottle 100 m into the lake, measuring the resulting sample with a thermocouple, and applying a cooling correction.

Information Contacts: David Stevenson, Open Univ.

September 1990 (BGVN 15:09) Cite this Report

Continued fumarolic activity; drop in crater lake water level

Evaporation of the crater lake was noted during fieldwork at the volcano the third week in July. Strong fumarolic activity continued within the lake, and a sulfur odor was detected near the summit, roughly 100 m above the lake.

Information Contacts: J. Barquero, E. Fernández, and V. Barboza, OVSICORI.

April 1991 (BGVN 16:04) Cite this Report

Ash ejection and lahars

A [phreatomagmatic] eruption at 1015-1025 on 8 May ejected small quantities of [ash, bombs, blocks, and mud, and produced small lahars]. Gray lahars with a sulfur odor traveled N down the Río Pénjamo and Azul systems, destroying the forest along the rivers and two small bridges, and cutting off access to the towns of Buenos Aires (12 km NE) and Gavilán. At the distal end of the lahars, 15 km from the summit, the deposits reached 2 m in thickness, and covered the surface for several hundred meters on both sides of the Pénjamo river channels. Following passage of the lahars, the rivers were milky and had high acidity. The eruption followed two smaller explosive events on 6 and 7 May, but no other seismic precursors were recorded.

Information Contacts: R. Barquero, ICE; J. Barquero and R. Sáenz, OVSICORI.

May 1991 (BGVN 16:05) Cite this Report

More details on 8 May eruption and deposits

The following, from the Univ Nacional, supplements last month's report from ICE.

A phreatic eruption on 8 May ejected lake sediments and ash, and produced small mudflows. The eruption followed several low-frequency earthquakes during the night of 6-7 May, and a low-frequency earthquake with a 155-second duration at 0811 on 7 May. Reports from residents of Dos Ríos de Upala (8 km NW) and from guards at Parque Nacional Rincón de la Vieja described an accompanying explosion and a 1-km-high light-colored plume with ash that traveled NW.

Seven low-frequency microearthquakes preceded the 8 May phreatic eruption. An earthquake that lasted 120 seconds, possibly associated with a small explosion, occurred 18 minutes prior to the eruption, and low-frequency tremor began 7 minutes before it.

The sound wave of the main explosion arrived at the seismometer (6 km SW) 6 seconds after the start of the eruption signal at 1017, and the instrument was saturated for 25 seconds. The subsequent 150-second signal was interpreted to record strong degassing and the initiation of mudflows. Low-frequency harmonic tremor was recorded for 30 minutes, gradually decreasing below detection limits. The main explosion produced a gray ash cloud, 5 km high, that was carried NW. Ash was deposited to 14 km NW from this (figure 1) and the approximately 10 small (columns

Figure 1. Map showing deposits from the 8 May 1991 phreatic eruption at Rincón de la Vieja. Site numbers correspond to cross-sections in figures 2 and 3, and table 1. Courtesy of OVSICORI.

Table 1. Field observations of 8 May 1991 mudflow deposits from Rincón de la Vieja. Sites correspond to locations in figure 1. Courtesy of OVSICORI.

Mudflows traveled down the N flank (along the Quebrada Azufrosa, and Río Pénjamo), destroying two small bridges and cutting off access to the towns of Buenos Aires (~12 km NE) and Gavilán. Several smaller mudflows traveled down tributaries to the Río Azul (also to the N). Erosion occurred predominantly between 1,500 and 500 m elevation. Field observations of the mudflow deposits were made at several sites (figures 2 and 3; table 1). Park guards reported small quantities of sediment transported by the Río Colorado (S flank), but no effects on the ecosystem were observed.

Figure 2. Cross-section of 8 May 1991 Rincón de la Vieja mudflow deposits near a bridge over the Río Azul. Site location is marked in figure 1. Courtesy of OVSICORI.

Figure 3. Cross-section of 8 May 1991 Rincón de la Vieja mudflow deposits near a bridge over the Río Pénjamo. Site location is marked in figure 1. Courtesy of OVSICORI.

Blocks (to 40 x 50 cm) with impact craters and ejected lake sediments were found near the summit during a 9 May visit. Acidity and sediment-fall had variable impacts on nearby vegetation, ranging to complete defoliation. Fumarolic activity continued, as evidenced by a strong sulfur odor, eye irritation, and breathing difficulties near the crater. Rain collected 3 km S had a pH of 3.85.

Seismicity declined to 9 low-frequency recorded earthquakes/day (9 May), with only sporadic (1-2/day) events on later days.

Information Contacts: E. Fernández S., Jorge Brenes M., V. Barboza M., and Tomás Marino H., Observatorio Vulcanológico y Sismológico de Costa Rica, Univ Nacional.

July 1991 (BGVN 16:07) Cite this Report

Seismicity and tremor

A total of 399 microearthquakes were recorded in July (figure 4) at a seismic station (RIN3) 6 km SW of the crater. Six hours of low- and medium-frequency tremor (1.3-3.2 Hz), were recorded in episodes 12 minutes to 3 hours long. Low-frequency earthquakes were also recorded, with durations that reached 175 seconds.

Figure 4. Daily number of earthquakes at Rincón de la Vieja, July 1991. Courtesy of OVSICORI.

Information Contacts: J. Barquero, E. Fernández, V. Barboza, and J. Brenes, OVSICORI.

August 1991 (BGVN 16:08) Cite this Report

Explosions eject ash and blocks

After reports of strong sulfur odors, geologists visited the summit area on 28-30 August. A sulfurous odor was noted at Copelares on the S flank (1,400 m elevation), during the evening of 28 August. An explosion was heard at 0151 the next morning, followed several seconds later by the sound of falling material. Examination of 29 August records from a seismic station 6 km SW of the crater (RIN3) showed that a small earthquake occurred at 0148:47, then a larger earthquake sequence lasting 7.5 minutes began at 0151:40, coinciding with the first audible explosion. As the ascent continued later that morning, traces of fresh ash were observed beginning at about 1,500 m elevation. Large quantities of ash and blocks, ranging from 15 to 75 cm in diameter, were found deposited in the summit area. Impact craters reached 120 cm in diameter and 35 cm deep.

Bad weather obscured the view of the crater floor, but several explosions were heard, and the largest, at 0930, rained very wet ash on the scientists. Near the crater, the smell of sulfur was very strong, making breathing difficult and stinging the eyes. Nearby vegetation was partially or completely dead. Rain collected at Copelares had a pH of 4.1.

On 30 August, scientists visited Ríos Azul and Pénjamo, which flow down the N flank from the crater area. Both rivers were gray-white with suspended sediment, which was also visible, but in lower concentrations, in the Ríos Colorado and Blanco on the S and SE flanks.

[On 6 September, strong fumarolic activity (jet engine noise) was seen in the active crater. During explosive events of May-August 1991 the ejecta was mainly composed of gray mud (sulfide-rich), lithics, and bread-crust bombs (~10% by volume).]

Information Contacts: J. Barquero and E. Fernández, OVSICORI; R. Barquero and G. Soto, ICE; Mario Fernández, Héctor Flores, and Sergio Paniagua, Univ. de Costa Rica.

February 1992 (BGVN 17:02) Cite this Report

Gas emission and sporadic phreatic eruptions

Gas emission has continued over the last several months, punctuated by sporadic phreatic eruptions. Fumarolic activity was concentrated on the active crater's E wall, producing a plume that occasionally reached 500 m height, smelling of sulfur, and irritating eyes and skin. The crater lake was gray, with yellow areas over bubbling points. Concentric and radial fissures, to 1 m wide and to >4 m deep, were found on the upper E, N, and NW flanks. The fissures were probably formed by partial collapse of the crater walls, especially on the E and NW flanks. Seven low-frequency earthquakes were recorded during February, down from a peak of 30 recorded 8 May 1991, associated with a large phreatic eruption. Abnormal seismicity was reported for several months after 8 May.

Information Contacts: E. Fernández, J. Barquero, V. Barboza, and R. Van der Laat, OVSICORI.

March 1992 (BGVN 17:03) Cite this Report

Small explosions and rumbling; earthquakes

Rumbling and small explosions were reported on several occasions in March. Portable seismometers at Proyecto Geotérmico Miravalles (35 km SE of the active crater) recorded small earthquakes, possibly associated with explosions, on 13-16, 20, and 26 March. A maximum of five events was recorded, on 16 March.

Information Contacts: G. Soto and R. Barquero, ICE; Guillermo Alvarado, GEOMAR, Christian-Albrechts-Universität, Germany; Mario Fernández, Univ. de Costa Rica.

May 1992 (BGVN 17:05) Cite this Report

Thermal activity from crater lake; occasional seismicity

The active crater lake (150-200 m diameter) was gray to dirty white during May fieldwork, with weak, intermittent bubbling. Fumarolic activity in the E part of the crater, where water was slightly greenish, was stronger than during February fieldwork. The activity, audible at the crater rim, produced a plume that rose more than 100 m (the height of the crater wall), and was visible several kilometers N. Crater-lake level had dropped about 30 cm since February, while the temperature remained at 37°C and the pH at 1.6. Small mats of sulfur were visible on the lake surface. Weak vapor emission began at several points along a fissure (first observed in February) near the SE and SW rim, with temperatures of 55°C and 60°C, respectively.

Six microearthquakes were recorded in May (at OVSICORI station RIN3, 5 km S). A 16-minute tremor episode (1-2.5 Hz) was recorded on 22 May.

Information Contacts: G. Soto, R. Barquero, and Guillermo E. Alvardo, ICE; Mario Fernández, Univ. de Costa Rica; E. Fernández, J. Barquero, and V. Barboza, OVSICORI.

June 1992 (BGVN 17:06) Cite this Report

Continued fumarolic activity

Fumarolic activity continued through June in the active crater, where it had fed a plume more than 100 m high during May fieldwork. Chemical analyses of water collected 13 May showed pH values of less than 3 in two of the three N-flank rivers sampled, and some enhancement in sulfate and chloride concentrations (table 2). A seismographic station 5 km SW of the crater (RIN3) registered seven low-frequency earthquakes in June.

Table 2. Chemistry of water collected 13 May 1992 from three rivers on the N flank of Rincón de la Vieja. Data courtesy of the Univ. de Costa Rica.

Information Contacts: E. Fernández, J. Barquero, and V. Barboza, OVSICORI; G. Soto, ICE; Mario Fernández, Univ. de Costa Rica.

August 1992 (BGVN 17:08) Cite this Report

Strong degassing; explosion seismicity

Strong degassing occurred in the active crater. Some seismic signals with durations of up to 3 minutes were recorded during August, interpreted by ICE geologists as probably associated with small or moderate eruptions from the hot crater lake. However, no deposits of mud were found on the crater rim during fieldwork on 28 August, so any deposition must have been confined to the interior of the crater. Parts of the rim rise >150 m above the lake. A UNA seismic station (RIN3) 5 km SW of the main crater recorded 11 low-frequency events in August, with brief (1-6 minute) periods of tremor at frequencies of <2.5 Hz.

Information Contacts: G. Soto and R. Barquero, ICE; E. Fernández, J. Barquero, and V. Barboza, OVSICORI.

September 1992 (BGVN 17:09) Cite this Report

Strong fumarolic activity; seismic swarm

Intense fumarolic activity continued from the SE inner wall of the active crater (figure 5). The emissions were rich in water vapor and sulfurous gases, emerging with a jet-engine sound and rising 350 m above the level of the crater lake in the absence of wind. Sulfur cones formed at the vents. Four main hot areas were evident in the crater lake (near the center, and on the NW, N, and NE sides), which was about 250 m in diameter. The lake was colored gray by the large amount of muddy sediment that it carried in suspension. Strands of mud and sulfur floated on the lake surface. Its temperature was measured by the Instituto Costarricense de Electricidad (ICE) at 52°C on 25 September. Of the 403 events of medium to high frequency recorded in September, 388 occurred between 1 and 10 September, the period in which the medium-frequency seismicity (>2.5 Hz) was recorded (figure 6). Sporadic low-frequency harmonic tremor was detected for periods of up to 2.5 minutes.

Figure 5. Topographic map of the Rincón de la Vieja complex, showing the site of the currently active crater. A national park building near the Las Pailas thermal area on the lower S flank is marked P.N. Contour interval, 200 m below 1,400 m elevation, 100 m at higher altitudes. Courtesy of ICE.

Figure 6. Number of seismic events per day at Rincón de la Vieja, recorded by a UNA seismic station (RIN3), 5 km SW of the main crater. Courtesy of UNA.

Information Contacts: E. Fernández, J. Barquero, and V. Barboza, OVSICORI; G. Soto and R. Barquero, ICE.

October 1992 (BGVN 17:10) Cite this Report

Degassing and minor seismicity

Degassing continued from the active crater. Seismic stations at the Proyecto Geotérmico de Miravalles (8-10 km from the active crater) continued to register 2-3 low-frequency volcanic events/day of <100 seconds duration. Vigorous seismicity had continued into early September, after seismic signals interpreted as probably associated with small to moderate eruptions from the lake were recorded in August.

Information Contacts: G. Soto and R. Barquero, ICE; E. Fernández, J. Barquero, and V. Barboza, OVSICORI.

January 1993 (BGVN 18:01) Cite this Report

Fumaroles; minor seismicity

Fumarolic activity continued in the E wall of the active crater. A seismic station (RIN3) 5 km SW of the crater registered 30 high-frequency shocks on 26 January. The same day at 1646, a M 3.0 earthquake occurred 4 km NNE of the main crater at 9 km depth. No significant tilt changes were observed during the most recent measurements in November 1992.

Information Contacts: E. Fernández, J. Barquero, R. Van der Laat, F. de Obadía, T. Marino, and R. Sáenz, OVSICORI; M. Martini, Univ di Firenze, Italy.

February 1993 (BGVN 18:02) Cite this Report

Gas plumes rise to 500 m; lake level drops

Fumarolic activity continued from the E wall of the active crater, with gas plumes rising 500 m. A strong smell of sulfur near the crater caused eye and skin irritation. Gas vents in the SE and SW parts of the crater had disappeared. Small collapses had occurred along the E and NE crater walls.

The level of the crater lake has dropped 1 m since last year. The light-gray colored lake had a temperature of 35°C in February and a pH of 1.6. The number of floating sulfur patches has decreased, and only one small bubbling area remains, producing very small intermittent bubbles.

Information Contacts: E. Fernández and J. Barquero, OVSICORI.

March 1993 (BGVN 18:03) Cite this Report

Hour-long earthquake swarm

The seismic station 5 km SW of the main crater recorded an hour-long swarm of 10 volcano-tectonic earthquakes (M <1.9) on 26 March. Fumarolic activity continued inside the crater and in the crater lake.

Information Contacts: G. Soto and R. Barquero, ICE; E. Fernández, J. Barquero, V. Barboza, T. Marino, R. Van Der Laat, F. de Obaldía, and R. Sáenz, OVSICORI.

April 1993 (BGVN 18:04) Cite this Report

Seismic activity continues

A seismograph about 5 km SW of the active crater recorded 28 microearthquakes and four high-frequency earthquakes in April (figure 7).

Figure 7. Seismic events/day recorded 5 km SW of the active crater of Rincón de la Vieja. Courtesy of OVSICORA.

Information Contacts: E. Fernández, J. Barquero, V. Barboza, and W. Jimenez, OVSICORI.

May 1993 (BGVN 18:05) Cite this Report

Seismicity continues

A total of 40 seismic events was recorded in May at a station 5 km SW of the main crater: 35 middle- and low-frequency events, and five high-frequency shocks (1.2-3 Hz). Volcanic tremor peaked on 20 May when 1.5 hours were recorded. Tremor was also detected 23, 25, 27, and 30 May for 11, 8, 3, and 8 minutes, respectively.

Information Contacts: E. Fernández, J. Barquero, F. De Obaldía, T. Marino, R. Van Der Laat, V. Barboza, and R. Sáenz, OVSICORI.

September 1993 (BGVN 18:09) Cite this Report

Crater lake level drops 10 m

A visit to the active crater on 6 October took place in bad weather, but scientists found strong fumarolic activity and a >10 m drop in lake level compared to September 1992. The lake level began changing after the 8 May 1991 phreatic eruption.

Low-frequency microseismic activity increased over the last 3 months, with three events in July, five events in August, and 93 events in September. Tremor was not reported in July, 165 minutes of tremor occurred in August, and no tremor was detected in September. Dry-tilt for the interval from November 1992 to July 1993 indicated an 11 µrad radial deflation; in contrast, the majority of intervals as far back as 1987 showed little or no change.

Information Contacts: E. Fernández, J. Barquero, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, and R. Sáenz, OVSICORI; G. Soto, Guillermo E. Alvarado, and Francisco Arias, ICE; Héctor Flores, Univ. de Costa Rica.

January 1994 (BGVN 19:01) Cite this Report

New seismic data, but still relatively quiet

During 1993 Rincón de la Vieja continued fumarolic venting in Rincón crater. Gas columns rose to 500 m high and contained sufficient gas concentrations to irritate the eyes and sting the skin of observers on the crater margin. The crater lake was clear gray in color, with clouds of suspended sulfur and several areas of discontinuous bubbling. It had a temperature of 35°C.

When seismically active, as in January and September 1993 (figure 8), both high- and low-frequency signals were common. A swarm of 25 high-frequency events took place on 26-29 January. On 26 March, 10 small-amplitude high-frequency events registered, their S - P (S minus P) times were

Figure 8. Seismic events at Rincón de la Vieja received at station RIN3, 5 km SW of the active crater, January 1993-January 1994 (October-December 1993 data presumed unrecorded). Courtesy of OVSICORI.

Information Contacts: E. Fernández, J. Barquero, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, and R. Sáenz, OVSICORI.

March 1994 (BGVN 19:03) Cite this Report

Subaqueous degassing; fractures surrounding SE crater rim

During March, Rincón de la Vieja continued fumarolic and seismic activity. The crater lake, which was 40 cm below the level seen in June 1993, had a temperature of 36°C. The lake had a clear gray color, although a fog of condensed gases hovering over the lake hampered visual observations. Visitors noted that vigorous, noisy fumaroles in the E crater wall produced enough sulfurous fumes to provoke coughing and irritate the eyes and skin. Fumes have also injured the already sparse vegetation adjacent to the active crater.

ICE researchers reported "sporadic and intermittent bubbling events (up to several meters in height and diameter) rising up from the center and SE portions of the warm lake, producing strong waves and noise, and giving a muddy-gray color to the lake." They also saw new, open fractures surrounding the SE crater rim.

In the interval February-March 1993, Rincón's seismic station registered an increase in events of low frequency (0.5-1.3 Hz) with durations [of] 25-150 seconds (figure 9). When previously seismically active, as in January and September 1993, both high- and low-frequency signals were common.

Figure 9. Seismic events at Rincón de la Vieja received at station RIN3, 5 km SW of the active crater, January-March 1994. Courtesy of OVSICORI.

Information Contacts: Gerardo J. Soto, Guillermo E. Alvarado, and Francisco (Chico) Arias, ICE; E. Fernández, J. Barquero, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, and R. Sáenz, OVSICORI.

April 1994 (BGVN 19:04) Cite this Report

Decreased seismicity

During April, the local seismic station received only 13 low-frequency events. In contrast, there were 283 low-frequency events during the previous month, the most so far this year. Neither the increase nor the decrease in seismicity were associated with any other observed changes.

Information Contacts: E. Fernández, J. Barquero, V. Barboza, and W. Jiménez, OVSICORI; G. Soto, Guillermo E. Alvarado, and Francisco (Chico) Arias, ICE; Héctor (Chopo) Flores, Univ. de Costa Rica.

September 1994 (BGVN 19:09) Cite this Report

Vigorous fumarolic activity

Fumarolic activity in the main crater remained vigorous during August and September. Preliminary processing of seismicity recorded by ICE with a portable digital station 2.2 km S of the crater during fieldwork in late August indicated several hundred low-frequency earthquakes beneath the crater, and background tremor-like activity. The preliminary interpretation is that the low-frequency seismicity is caused by hydrothermal circulation among a shallow magma body, aquifers, and the lake system. The OVSICORI-UNA seimic station (5 km SW of the active crater) registered 15 high-frequency low-magnitude events during September.

From the village of México (40 km NE), early morning observations during late September and early October by an ICE geologist revealed a steam-rich gas column rising up to 1 km above the crater. This is higher than the 300-400 m estimated in March.

Information Contacts: E. Fernandez, J. Barquero, V. Barboza, R. Van der Laat, T. Marino, F. de Obaldia, and L. Carvajal, OVSICORI; G. Soto, W. Taylor, F. Arias, G. Alvarado, and R. Barquero, ICE; Mauricio Mora, Univ. de Costa Rica.

October 1994 (BGVN 19:10) Cite this Report

Thirty-one small high-frequency events

Seismic station RIN (5 km W of the active crater) received 31 events of high-frequency. The events were only detected locally, they had Richter magnitudes of less than 1, and S minus P times of less than 2 seconds. For comparison, during April, the local seismic station received only 13 low-frequency events. In contrast, there were 283 low-frequency events during the previous month, the most reported so far this year.

Information Contacts: E. Fernández, J. Barquero, and V. Barboza, OVSICORI.

November 1994 (BGVN 19:11) Cite this Report

Vigorous fumarolic activity continues

The fumarolic activity in the main crater that remained vigorous during August and September, continued in November. A seismic record made by ICE in November suggested seismo-volcanic activity of low frequency and magnitude located at very shallow depths beneath the crater.

Information Contacts: E. Fernández, J. Barquero, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, and R. Sáenz, OVSICORI; G. Soto, Guillerma E. Alvarado, and Francisco (Chico) Arias, ICE.

December 1994 (BGVN 19:12) Cite this Report

Minor seismicity

During December 1994, Rincón de la Vieja was seismically active. The local station (5 km SW of the active crater) registered three small-amplitude events on 19 December and, at 2121, a fourth of Richter magnitude 3.0. The last event had a focal depth of 23 km and an epicenter 4.5 km NE of the active crater. One event took place each day on 23, 24, and 25 December; these events each had an S -P delay in the range of 1.9 to 2.5 seconds.

Information Contacts: OVSICORI.

January 1995 (BGVN 20:01) Cite this Report

Ongoing low-frequency seismic signals and fumarolic venting

During January Rincón de la Vieja continued fumarolic venting from the main crater. ICE reported that they continued to record seismic signals of low-frequency and magnitude at the volcano. They interpreted the signals as seismo-volcanic activity at shallow depth beneath the main crater.

Information Contacts: ICE.

April 1995 (BGVN 20:04) Cite this Report

Description of the crater lake and fumaroles

The remote Rincón de la Vieja volcanic complex continues to display unsettled seismic and fumarolic activity. OVSICORI-UNA reported that during April fumarolic venting continued from the W wall, creating noise audible from the crater's rim. Escaping gases stung the skin. Radial fractures encircled the crater on its NE, N, and NW sides.

G. Soto (ICE), Jean-Philippe Rancon, and Gorges Boudon climbed the volcano on 1 May and reported that the lake contained a scum of floating sulfur and was pale turquoise in color. No lake temperature measurements were made but the entire surface steamed slightly. In contrast to a previous visit in March 1994, the lake level seemed significantly higher, although the amount has yet to be quantified from photographic records; zones of bubbling (previously several meters across) were absent.

Fumaroles on the crater's inner SE wall were quite active and fumed noiselessly. Gas plumes, clearly visible from the volcano's N flank, rose up to 100 m above the crater before being blown by the wind. Small, steam-rich fumaroles adjacent to concentric fractures surrounded the crater, typically near the 1,640 m contour. These fumaroles were also active last year.

At least two other noteworthy fumaroles, expelling steam and sulfurous gases, sit on the N flank (along the valley called Quebrada Azumicrorada at around 1,200- and 1,300-m elevation). In clear weather, these fumaroles are visible from local villages and residents stated that they had been active for the past several years.

Information Contacts: Erick Fernandez, Vilma Barboza, and Jorge Barquero, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Gerardo J. Soto, Oficina de Sismologia y Vulcanologia del Arenal y Miravalles: OSIVAM; Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica; Jean-Philippe Rancon, BRGM, Orleans, France (presently at USGS Cascades Volcano Observatory, 5400 MacArthur Blvd., Vancouver, WA 98661-7095 USA); Georges Boudon, Observatoires Volcanologiques, Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252 Paris 05, France.

September 1995 (BGVN 20:09) Cite this Report

Seismic activity continues at a rate of tens of events per month

The seismic receiver at the remote Rincón de la Vieja volcanic complex (RIN3) is located 5 km SW of the active crater. During August it registered 42 events at frequencies below 1.5 Hz; during September, 28 events with frequencies below 2.5 Hz.

Information Contacts: E. Fernandez, E. Duarte, R. Sáenz, W. Jimenez, and V. Barboza, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

October 1995 (BGVN 20:10) Cite this Report

New eruption; lahars damage a bridge and lead to evacuations

A new phreatomagmatic eruption followed three months of declining seismicity. During 1995 the number of local earthquakes peaked in July and then progressively decreased (figure 10). Prior to the eruption, during October, OVSICORI-UNA reported that park rangers who ascended to the main summit saw increased degassing and noted the appearance of fumaroles along cracks at the E and NE crater margins. Rangers described the crater lake's color as green and the smell as strong and sulfurous.

Figure 10. The number of monthly earthquakes at Rincón de la Vieja volcanic complex recorded 5 km SW of the active crater (station RIN3), January-October 1995. The seismic system failed to operate on 29 October; the three events recorded during the rest of the month were all of low frequency (

ICE described the eruption as phreatomagmatic, beginning at 1504 on 6 November, and climaxing on 8 November with 25 explosions. They noted the ash-bearing and steam-rich columns rose to 1 and 4 km, respectively, above the crater. Ash blew WSW; medium- to fine-grained ash reached up to 30 km from the volcano (Santa Rosa National Park).

According to ICE, on 9 November the eruption entered a steam-rich phase. Columns typically rose 200 m, but sometimes as much as 1.5 km after some steam explosions.

During the course of the eruption, ballistic ejecta were thrown over a zone extending to ~1 km N. Ejecta formed lahars that followed two key rivers (Penjamo and Azul rivers) and their tributaries. Heavy rains beginning on 10 and continuing on 11 November triggered secondary lahars and associated floods; a bridge 7 km N of the crater (Penjamo bridge) was damaged but not destroyed, interrupting traffic flow. During this episode, lahars along a tributary of the Penjamo river produced a gully 8-m deep and 25-m wide, isolating some inhabitants.

Initial inspections of ash and the lahar matrix indicated that they mainly consisted of hydrothermally altered fragments, lake-sediment mud, and vesiculated glassy andesite fragments.

Some residents living near the volcano were evacuated to a safe village 9 km NW of the crater. News reports on 8 November by both Associated Press and Deutsche Presse-Agentur stated that about 100 families were evacuated. Two days later Enrique Coen reported relocation of 300 families.

Information Contacts: E. Fernandez, E. Duarte, and V. Barboza, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; G.J. Soto, Oficina de Sismologia y Vulcanologia del Arenal y Miravalles: OSIVAM, Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica; Enrique Coen, Departamento de Fisica, University Nacional, Heredia, Costa Rica; Associated Press; Deutsche Presse-Agentur.

December 1995 (BGVN 20:11) Cite this Report

Additional details about the 6-10 November eruption

An eruption on 6 November 1995 followed increases in fumarolic activity and a several-month long increase in local earthquakes and tremor (figures 11 and 12). Park rangers who visited the summit at the start of October noted increased fumarolic activity and witnessed landslides down the main crater's walls. Strong sulfur smells were noted W-SW of the volcano on multiple occasions in the days prior to 6 November (figure 13).

Figure 11. Rincón de la Vieja's monthly totals for tremor and low-frequency seismicity, January-September 1995. Courtesy of OVSICORI-UNA.

Figure 12. Rincón de la Vieja's seismicity, 1-13 November 1995. An eruption began on 6 November. Courtesy of OVSICORI-UNA.

Figure 13. Map of NW Costa Rica showing key features associated with Rincón de la Vieja's 6 November 1995 eruption. Courtesy of OVSICORI-UNA.

The seismic receiver (RIN3) sits 5 km SW of the active crater. Although the OVSCICORI-UNA seismic system failed on 29 October (and possibly other times during the month), it functioned reliably again after the 31st. Low-frequency events gradually increased during 1-6 November (figure 12), followed by a modest decline. High-frequency events were only registered after 3 November. Tremor was absent prior to the 6 November eruption.

OVSCICORI reported that the first phase of the eruption consisted of vapor with subordinate ash in a discharge lasting 2 minutes. Later, vigorous fumarolic activity led to many hours of constant tremor. Only two more clear eruptions followed in the initial 17 hours of venting, but others followed in subsequent days. The eruption climaxed on the morning of the 8th, when columns reached 3.5 km altitude. Fine ash blew W and NW; larger blocks and tephra were confined to within ~1 km and the area of heavy ashfall reached ~5 km away (figure 13).

During some phases of the eruption, lahars flowed down the Azul and Penjamo rivers and an interfluvial ravine called the Quebrada Azumicrorada (figure 13). Upper reaches of these drainages sustained up to 6 m of erosion. Lahars on the 7th were cooler and more water-rich than those on the 8th. In addition to previously reported damage, on 8 November lahars shut down some communications systems.

At 0900 and 1130 on 8 November OVSICORI scientists visited the summit area and saw impact craters as large as 2 m in diameter; the craters were produced by 0.5-1.0 m diameter blocks, some of which were still warm to the touch. The scientists also saw ongoing phreatic eruptions escaping from a vent adjacent to the crater lake.

At 0411 on the 9th a shock wave was felt 25 km SE in the city of Liberia; the related outburst was seen from the N flank, where residents witnessed incandescent block ejections.

Amplitudes on the seismic recorders regularly peaked at over 30 mm on 6-9 November. The highest amplitudes, on 7-9 November, reached nearly 60 mm. Amplitudes decreased the morning of 9 November; following the eruption (10-14 November) amplitudes generally remained under 10 mm with infrequent spikes to ~20 mm and a few rare spikes to 30 mm. Tremor decreased by an order of magnitude on 10 November and it dropped to <1 hour/day on 13 November.

During fieldwork in early December, G. Soto (ICE) and G. Boudon (IPG) inspected the near-source region. For a radial distance of ~1 km from the crater they saw a deposit consisting of muddy ash, lapilli, and blocks. These reached 40 cm thick on the crater's southern outer rim at a point 150 m from the inner rim. The deposit's thickness and grain size decreased rapidly with distance, such that at 600 m SW of the crater the deposit was only 7 cm thick. The deposit's basal zone was enriched in fine grained, muddy-looking material, but throughout the deposit there occurred lustrous black juvenile clasts. Over ~1 km² of the upper surface of the deposit, there lay a blanket consisting of (a) dense, quenched blocks, (b) breadcrust bombs with notably vesicular cores, and (c) some highly vesiculated fragments. On 8 December at points 5 and 8 km from the summit, the Penjama and Blanco rivers, respectively, still ran milky and were slightly acidic in taste. That same day, the scientists saw only fumarolic activity. Although scientists looked for a lake in the depths of the crater, they failed to gain a clear view there.

Reference. Boudon, G., Rancon J.-P., Kieffer, G., Soto, G.J., Traineau, H., and Rossignol, J.-C., 1995, Estilio eruptivo actual del Volcan Rincón de la Vieja: evidencias de las productos de las erupciones de 1966-70 y 1991-92: Rothschildia, 2 (2): 10-13, Area de conservacion de Guanacaste, Costa Rica.

Information Contacts: E. Fernandez, E. Duarte, R. Sáenz, W. Jimenez, and V. Barboza, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Georges Boudon, Institut de Physique du Globe de Paris, 4, Place Jussieu, 75252, Paris Cedex 05, France.

January 1996 (BGVN 21:01) Cite this Report

Eruption on 11-13 November followed by decreasing seismicity

The eruption that began on [6] November had ended by 13 November (BGVN 20:11/12), yet somewhat elevated seismicity (4 events/day) prevailed through late November. Although the seismic system (RIN3, 5 km SW of the active crater) later failed (all or partly inoperative, 3 December-3 January), it received low-frequency events during most of 1-10 December at the rate of 1-3 events/day, and on 6 December it recorded eight events. During January, RIN3 registered near background levels of seismicity: 8 events/month, consisting of two at low frequencies and six at high frequencies.

Information Contacts: Rodolfo Van der Laat, Vilma Barboza, Erick Fernández, Jorge Barquero, Franklin de Obaldia, Tomás Marino, and Rodrigo Sáenz, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

March 1996 (BGVN 21:03) Cite this Report

Mild seismicity continues in February

Several small-to-moderate eruptions took place in early November 1995 (BGVN 20:10 and 20:11/12). Mild seismicity continued after the eruption; during February seismic station RIN3, located 5 km SW of the active crater, registered seven microseisms (six low-frequency, one high-frequency). These microseisms were only detected locally.

Information Contacts: Erick Fernández, Elicer Duarte, Vilma Barboza, Rodolfo Van der Laat, and Enrique Hernandez, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Gerardo J. Soto, Oficina de Sismolog¡a y Vulcanolog¡a, Departamento de Geolog¡a, Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica.

May 1996 (BGVN 21:05) Cite this Report

Seven minor seismic events

During May seismic station RIN3 registered a total of seven events: two of high frequency and five of low frequency.

Information Contacts: Erick Fernández, Elicer Duarte, Vilma Barboza, Rodolfo Van der Laat, and Enrique Hernandez, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

June 1996 (BGVN 21:06) Cite this Report

Six-fold seismic increase over previous months in 1996

The local seismic station (RIN3, located 5 km SW of the active crater) registered a total of 50 events, a 6-fold increase over any previous month in 1996. These events were only detected at this seismic station.

Information Contacts: Erick Fernández, Elicer Duarte, Vilma Barboza, Rodolfo Van der Laat, and Enrique Hernandez, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

August 1996 (BGVN 21:08) Cite this Report

Small local earthquakes

Seismic station RIN3, located 5 km SW of the active crater, detected local microseisms. During July there were 27 events; during August, 16 events. For comparison, the near background levels of seismicity during January 1996 consisted of 8 events/month.

Information Contacts: E. Fernández, E. Duarte, V. Barboza, R. Van der Laat, E. Hernandez, M. Martinez, and R. Sáenz, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

January 1997 (BGVN 22:01) Cite this Report

Sulfur-bearing steam, mass wasting, and acid rain in September

A visit to the crater during September 1996 revealed vegetation killed by the phreatic eruption of November 1995 (BGVN 20:10 and 20:11/12), by mass wasting in its aftermath, and by continued acidic rainfall. Although some plants were beginning to return, most suffered visible damage such as scalded leaves. The crater area continued to smell of sulfur and the loud noise of escaping steam could be heard from the crater's margin.

Information Contacts: E. Fernández, E. Duarte, V. Barboza, R. Van der Laat, E. Hernandez, M. Martinez, and R. Sáenz, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

May 1997 (BGVN 22:05) Cite this Report

Conspicuous fumaroles and plumes persist

During April, fumarolic activity remained in the E and S parts of the main crater. In the latter location, escaping gases hissed like a pressure cooker and were audible from the crater rim. Gas columns rose up to 200 m high. Adjacent to the crater, visitors smelled sulfur gases and their throats, eyes, and skin became irritated. Some of the plants damaged during November 1995 showed new signs of recovery. Although the seismic station (RIN3, located 5 km SW of the active crater) remained out of service during May, earthquake counts numbered five events in December 1996 and 24 in January 1997.

Information Contacts: E. Fernandez, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, W. Jimenez, R. Sáenz, E. Duarte, M. Martinez, E. Hernandez, and F. Vega, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

September 1997 (BGVN 22:09) Cite this Report

Variable but modest seismicity during June-September 1997

OVSCORI-UNA reported that over the last several months earthquake totals were as follows: June, 2; July, 43 (including 24 microseisms); August, 20; and September, 100 (including 45 microseisms). Gerardo Soto (OSIVAM) spoke with park rangers who said there had been several rockslides along the crater's NE and N inner slopes during the first half of 1997. Some of these slides created small islands in the lake.

Information Contacts: E. Fernandez, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, W. Jimenez, R. Sáenz, E. Duarte, M. Martinez, E. Hernandez, and F. Vega, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; G.J. Soto, Oficina de Sismologia y Vulcanologia del Arenal y Miravalles (OSIVAM), Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica.

March 1998 (BGVN 23:03) Cite this Report

Phreatic eruptions on 15-17 February thrust steam to 2 km

Beginning at 1428 on 15 February, Rincón de la Vieja volcano discharged phreatic eruptions from the main crater. Ten eruptions took place in the first 15 hours of activity; only two followed in the subsequent 13 hours. During the course of the outburst subsidiary fumarolic activity also became more vigorous; it remained elevated until 18 February.

During 15-17 February numerous steam plumes rose hundreds of meters above the volcano. On 17 February one outburst sent a steam plume to a height of 2 km above the crater. This plume was seen by residents on the N and NE flanks of the volcano. A dozen eruptions around this time were small and lacked associated mudflows. An exception, at 0514 on 16 February, produced a modest mudflow that traveled about 9 km/hour and left a capping deposit of mud 30-cm thick in the upper reaches of the Pénjamo and Azul rivers. Rivers had been low in the region, attributed to the El Niño phenomena, with the result that the mudflow was relatively dry. The mudflow had a large impact on local fish and other stream organisms. Sediment from the mudflow was found 12.3 km from the main crater.

Inspecting the 16 February deposit near the summit on 1 March, scientists inferred from the scorching, burning, and other damage to vegetation on the NE flanks that there must have been several smaller eruptions around that time as well. Mudflows failed to develop due to the paucity of surface water in local drainages.

The 1 March visit also revealed the lake's temperature, 48°C, its color, light gray, the presence of suspended sulfur in the lake, and a haze of condensed gases above the lake. An outgassing fumarole on the SW wall made loud hissing noises (similar to gases exiting a high pressure valve) audible from the crater's rim. Columns of gas rose about 200 m above the crater before being blown E. Those inspecting the scene noted strong sulfurous odors, and experienced irritated skin and eyes. The material erupted was uniformly fine- to medium-grained, lacking either bombs, blocks, or impact craters. This contrasted with deposits left by previous eruptions in 1991 and 1995.

The local seismic station (RIN3) lies 5 km SW of the active crater. The station registered microearthquakes as follows: during January, 18 (including 3 of high frequency and 9 of low frequency); during February, 48 (including 1 of high frequency, 21 of low frequency); during March, 7. In assessing their records of the 48 February microearthquakes, seismologists recognized 20 eruptions including 11 comparatively high-intensity phreatic eruptions mainly registered on 15-18 February. Banded tremor occurred on 15 and 16 February during the main eruptive interval; the tremor prevailed for a total of ~6.5 hours. Low in frequency, the tremor had amplitudes that ranged between 1.0 and 37 mm. The larger amplitude registered during the eruption's initial phase, at 1428 on 15 February. Tremor amplitudes later declined to the 1-4 mm range. As with the 1991 and 1995 eruptions, seismic precursors were absent.

Information Contacts: E. Fernandez, V. Barboza, R. Van der Laat, R. Sáenz, E. Duarte, E. Malavassi, T. Marino, M. Martinez, and E. Hernandez, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Mauricio Mora Fernandez, Sección de Sismologia, Vulcanologia y Exploración Geofisica, Escuela Centroamericana de Geología, Universidad de Costa Rica, P.O. Box 35-2060, San José, Costa Rica.

June 1998 (BGVN 23:06) Cite this Report

Non-eruptive and relatively quiet seismically during April-June

The volcano has been relatively quiet since phreatic eruptions on 15-17 February sent plumes to 2 km. During April, May, and June the local seismic system registered 3, 27, and 17 events, respectively. The April seismic events were of unspecified type. The May seismic events included eight of high frequency; in addition, one hour of low-frequency tremor took place. Seismicity during June included some low-frequency events.

Information Contacts: E. Fernandez, V. Barboza, E. Duarte, R. Sáenz, E. Malavassi, M. Martinez, and Rodolfo Van der Laat, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

June 1999 (BGVN 24:06) Cite this Report

1.5-year record of seismicity and eruptions through May 1999

Seismicity during 17 months through April 1999 (figure 14) showed pronounced peaks at over 100 events/month in one parameter, microseisms, during September- October 1998. Otherwise, relative quiet prevailed; microseisms, high-frequency, and low-frequency events all generally took place fewer than 20 times/month. Tremor was nearly absent during roughly half the months of 1998 and in 1999 during January, February, and April. Months with 2-10 hours of tremor included February, August, September 1998 and March and May 1999.

Figure 14. Selected seismic parameters at Rincón de la Vieja during January 1998-May 1999. The arrows indicate months with detected eruptions (mainly in February 1998, a month with ~10; the other indicated months with fewer than 2). Courtesy of OVSICORI-UNA.

In March, fumarolic activity continued on the NE, S, and SW walls of the main crater. The lake had a gray color and contained suspended particles of sulfur. The temperature of the lake was 35.5°C.

During May, the main crater's N-flank fumarolic activity fluctuated in temperature between 68°C and 92°C. The lake in the crater was light blue with particles of sulfur, and a temperature of 37°C. On the S and N walls, there were columns of gases that irritated eyes and skin.

Information Contacts: E. Fernandez, V. Barboza, E. Duarte, R. Sáenz, E. Malavassi, M. Martinez, and R. Van der Laat, T. Marino, and E. Hernandez, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Wendy Perez Fernandez, Seccion de Seismologia, Vulcanologia y Exploracion Geofisica, Escuela Centroamericana de Geologia, Universidad de Costa Rica, POB 35-2060, San José, Costa Rica.

March 2000 (BGVN 25:03) Cite this Report

Non-eruptive through November 1999 but with ongoing tremor

The noisy escape of fumarolic gases continued at Rincón de la Vieja during June-November 1999. A summary of monitoring data appears in table 3. During August the crater floor became covered with a shallow ephemeral lake, covering the fumaroles there. Plumes then rose less than 100 m above their fumarolic sources. The active crater lake, with a sky-blue color, had a temperature of 36°C; the maximum measured fumarole temperature was 70°C.

Table 3. Geotechnical data at Rincón de la Vieja, July-November 1999. Seismic data recorded at station RIN3, 5 km SW of the active crater, includes microseisms who's amplitudes were under 5 mm, and those volcano-tectonic (VT) earthquakes with S minus P arrival times under 1.5 seconds (i.e. focused near the volcano). The reported tremor durations were sums of discontinuous segments, and were low-frequency (below 2 Hz). Courtesy of OVSICORI-UNA.

Information Contacts: E. Fernandez, E. Duarte, V. Barboza, R. Sáenz, E. Malavassi, R. Van der Laat, T. Marino, J. Barquero, and E. Hernández, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

November 2001 (BGVN 26:11) Cite this Report

Fumarolically active through August 2001

During March 2000 through at least August 2001, fumarolic and seismic activity continued at Rincón de la Vieja. Fumarolic gases often irritated the eyes, skin, and throat.

On 1 March 2000 the crater lake was blue, with sulfur particles in suspension, a constant surge, and a temperature of 37°C. Compared to a visit in September 1999, the level of the lake was higher and the bubbling in the SW part had ended. The fumaroles on the NE (91°C) and SW walls were no longer steaming. The fumaroles on the NE flank (89°C) were steaming slightly. The edge of the crater displayed concentric 50-m-long and 40-cm-wide cracks.

During October 2000, the lake was gray with a high water level, sulfur particles floating on the surface, evaporation, and a temperature of 44°C. Fumarolic activity was observed in the SW and N wall of the main crater. The fumarolic area of the N flank (60°C) was steaming slightly, and sublimate deposition occurred.

During July 2001, OVSICORI-UNA reported that the level of the lake had descended ~2 m since mid-March. The lake was gray in color with sulfur particles floating on the surface; vigorous evaporation made observation of its bottom difficult, and its temperature stood at 58°C. In the SW wall there were small areas sliding towards the lake, and a new noisy fumarole appeared on the S wall. The fumaroles on the NE and SW walls remained active, producing gas columns that reached up to 300 m. The columns, often visible from the N and NW flanks, were blown by predominant winds towards the W and SW flanks. Low-frequency events and microearthquakes increased during June and August 2001. A summary of earthquakes at Rincón de la Vieja appears in table 4.

Table 4. Summary of earthquakes at Rincón de la Vieja during May 2000 to August 2001, registered by a seismograph at a station located 5 km SW of the main crater. The reported earthquakes include microseisms with amplitudes under 5 mm. The reported tremor durations were sums of discontinuous segments and were of low frequency (below 2 Hz). Missing months indicate that no data were available at the time of report preparation. Courtesy of OVSICORI-UNA.

General References. Barquero, J., and others, 1978-1986, Estado de los Volcanes de Costa Rica (15 annual or semi-annual reports): Boletín de Vulcanología, nos. 2-13 and 15-17.

Garcia, M.O., and Malavassi, E. (eds.), 1983, Memoir, USA-Costa Rica Joint Seminar in Volcanology, San José, January 1982: Universidad Nacional, Heredia, 155 p. (18 papers).

Information Contacts: Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

October 2007 (BGVN 32:10) Cite this Report

Continued fumarolic activity; sulfur floating in the lake

During September 2006 through at least May 2007, low-level fumarolic and seismic activity continued at Rincón de la Vieja. At the edge of the crater, Fumarolic gases often irritated the eyes, skin, and throat.

During September 2006, the level of the lake was high, with convection cells and particles of sulfur floating on the surface. The lake displayed yellow color with minor evaporation and a temperature of 39°C. Fumarolic activity was occurring in the S wall and SW part of the crater. Columns of gases rose above the edge of the crater and were carried by the predominant winds toward the W and SW. The fumaroles on the N side produced only low-level emissions.

By April and May 2007, the level of the lake had descended some 50 cm with respect to September 2006. The lake color turned to gray with minor evaporation. In the S, there were particles of sulfur floating on the surface and a temperature of 45°C. The fumarolic activity on the SW wall displayed low levels of gas emission and rich sulfur depositions. The fumaroles of the N side were inactive.

Information Contacts: E. Fernández, E. Duarte, R. Van der Laat, M. Martinez, W. Sáenz, V. Barboza, Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/).

July 2008 (BGVN 33:07) Cite this Report

Tremor durations from minutes to over an hour during June-April 2008

Activity at Rincón de la Vieja was last reviewed in May 2007 (BGVN 32:10), when low-level fumarolic activity was noted. During June 2007, the seismographic station 5 km to the SW of the crater registered seven low-frequency earthquakes and three low-frequency tremors. The first tremor occurred on 12 June and lasted 2 hours and eight minutes; the second and third occurred on 27 and 28 June and they lasted 37 minutes and 38 minutes, respectively.

The July earthquake activity was consistent with June; 6 low frequency quakes were recorded. Again, tremor activity occurred on 28 and 29 July, the first lasted 35 minutes and the second lasted 17 minutes. Little activity was noted during August and September, and October activity consisted only of tremors. On 23 October, the tremor lasted 37 minutes, and on 24 October it lasted 25 minutes.

No significant seismicity was recorded during the first three weeks of November. The seismic recording instrument went out of service from 22 November through December and January.

During February, 2008, 44 low-frequency earthquakes were registered in two groups; the first on 6 and 7 February and the second between 17 and 23 February.

Technical difficulties in March precluded a complete record of seismic activity; however, when recording was available, 116 low frequency earthquakes were noted.

Technical problems persisted in April, however during the first part of the month there was a low-frequency earthquake and 1.16 hours of tremor. Two deep earthquakes were also noted; the first registering M 3.5 at a depth of 11 km and the second M 2.9 at a 25 km depth.

Information Contacts: E. Fernández, E. Duarte, R. Van der Laat, M. Martinez, W. Sáenz, V. Barboza, Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/).

April 2011 (BGVN 36:04) Cite this Report

Fumarolically active but non-eruptive through January 2011

Low-frequency earthquakes and tremor were reported at Rincón de la Vieja during the first half of 2008 (BGVN 33:07). Since then, Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA) had issued intermittent reports of activity through January 2011. Those reports are summarized in the following sections, with much of the discussion centered around fumaroles and behavior of the geothermally warmed lake in the active crater. Occasional, typically small phreatic eruptions had occurred here in past years, for example in the 1990s (eg., BGVN 21:02, 21:03, 22:01, and 23:03) but were absent in the current reporting interval (last half of 2008 through January 2011).

August 2008. OVSICORI-UNA reported that the level of the lake was at a high level, with a bluish color, generated convection cells with evaporation, and had sulfur particles visible on it's surface. Sulfur deposition and fumarolic activity continued along the SW wall.

March 2009. In mid-March 2009, scientists visited the S and SW flank, collected samples, and noted some temperatures of 75-78°C. Because the visit occurred during the dry season, most areas encountered were dry. The scientists examined an area of acidification to the W of Von Seebach crater, ~3 km SW of the active crater. Strong winds common in that direction sometimes carried volcanic gases. Consequently, most of this narrow expanse only contained patches of grassland and shrubs that barely covered the rocky surface.

October 2009. OVSICORI-UNA reported that seismographic station RIN3, located ~5 km SW of the main crater, registered volcano-tectonic events and tremor lasting for minutes.

Weak ongoing fumarolic activity during 2010 through January 2011. OVSICORI-UNA reported that the level of the crater lake remained high during 2010, with constant evaporation. Geochemical, seismic, and deformation data did not show significant changes in physico-chemical parameters during 2010. The changing color of the lake, from blue to gray, was attributed to intense rains and fumarolic activity in the crater.

Later reporting. Reports during 2010 through at least January 2011 described fumarolic activity along the S and SW walls of the crater, with sulfur deposition and moderate gas discharge. The lake remained a gray color, with sulfur particles in suspension. Figure 15 shows a photo taken in April of the crater looking at the SW wall with fumarolic activity along with sulfur deposition. In April 2010, OVSICORI-UNA reported that the temperature of the lake was 49°C. A fumarole sometimes seen active along the N flank had stopped discharging gas.

Figure 15. Photo of the active crater lake of Rincón de la Vieja on 29 April 2010 showing yellow sulfur deposits and fumarolic activity along the SW wall of the crater. This kind of activity was typical throughout the reporting interval (last half of 2008 through January 2011). Photo by E. Fernandez, OVSICORI-UNA.

OVSICORI-UNA reported that 2010 was unusual in that four domestic volcanoes were active: Arenal, Poás, Turrialba, and Rincón de la Vieja. Irazú was comparatively inactive (see separate report in this issue of the Bulletin).

Information Contacts: E. Fernández, W. Sáenz, E. Duarte, M. Martínez, S. Miranda, F. Robichaud, T. Marino, M. Villegas, and J. Barquero, Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/).

January 2016 (BGVN 41:01) Cite this Report

Explosive eruptions resume in August 2011 after 13 years; intermittent activity through 2015

Costa Rica's Rincon de la Vieja volcano, part of a large volcanic complex in the Guanacaste Range, has had intermittent phreatic explosions with historical observations dating back to 1765, often separated by decades of inactivity. Major eruptions in the mid-1990s caused large mudflows and significant damage to the local communities. Since that time the volcano has presented persistent subaerial and subaqueous fumarolic activity around the crater lake, but otherwise has been quiet. Costa Rica's Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA) monitors the volcano, and after the 2011 eruptive episode (discussed below) installed new monitoring equipment, in conjunction with Observatorio Sismológico y Vulcanológico Arenal-Miravalles del Instituto Costarricense de Electricidad (OSIVAM-ICE), which also tracks events at the volcano.

The first explosions since 1998 began on 22 August 2011. This was followed by several more phreatic eruptions in September 2011. Explosions were again reported in February, April, and October 2012. No explosions were reported in 2013, although unusual seismic activity was recorded. In 2014, explosions were reported in September and October. Increased seismic activity and phreatic eruptions again occurred in June, August, and October 2015. There were no VAAC reports or MODIS thermal anomalies reported between January 2011 and December 2015.

Activity during 2011. Activity in early 2011 consisted of mild underwater and surface fumarolic activity in the SW and S wall of the active crater, and a moderate level of emissions. The acidic lake had a light gray color with sulfur particles in suspension. This was characteristic of conditions at the volcano over the previous 13 years.

Conditions changed on 22 August 2011, when a local guide reported seeing a column of dark material erupt 5 m high above the lake. Another eruption was observed on 9 September by a local guide; the fine sediment from that explosion was deposited on the N rim of the crater. On 16 September an eruption was heard by local guides around 1400, and an ash plume rose above the crater and deposited material 200 m downhill from the N edge of the crater. Heavy rain later in the day eroded most of the new material into the nearby Zanjon and Azufrada streams. The volcanic material caused changes in water quality along the main creeks and major rivers north of the volcano. Dead fish were noted the next day in the town of Buenos Aires de Upala (7.5 km N) and Birmania de Upala (18 km NNE). On the morning of 23 September, residents of Dos Rios de Upala (10 km NW) observed light gray ash on the northern edge of the crater from an eruption during the previous night.

OVSICORI-UNA scientists visited during 27-29 September 2011 to confirm the eruption and document its effects. Streams flowing north from the crater contained deposits up to 15 cm thick of fresh volcaniclastic sediment from the eruptions (figure 16) at least 2 km away. A thin layer of ash was also observed on nearby vegetation.

Figure 16. Freshly deposited sediment (10-15 cm thick) in a stream bed 2 km N of the crater from the eruptions of September 2011 at Rincon de la Vieja. Photo by Eliecer Duarte, 27 September 2011. Courtesy of OVSICORI-UNA (September 2011 Monthly Bulletin).

The scientists observed that the lake was actively convecting, and grayish in color due to the suspended sediments (figure 17). They also observed a small eruption from a cell at the center of the lake that rose about 1 m. The fumarolic activity was concentrated in the S and SW parts of the inner crater wall, and plumes of steam and gas rose above the eastern edge of the crater.

Figure 17. The acidic lake of Rincon de la Vieja volcano on 27 September 2011 during a visit by OVSICORI-UNA scientists after eruptive activity began again in August. Note the gray color indicating abundant suspended sediment, and the convection cell at the center of the lake. View is looking north from the south rim. Courtesy of OVSICORI-UNA (Field Report 25-27 September, 2011).

Several hot springs are located on the N flank. Readings from a thermometer installed near the hot springs indicated a systematic and substantial increase in water temperature between August and September 2011, consistent with the reports of renewed eruptive activity. The lake is highly acidic (pH

No eruptions were reported in October 2011, but when scientists visited again on 23 November the lake level remained high, was light gray in color, and had a temperature of 34°C. Poor weather prevented visual observation, but rumbling noises from the lake suggested a small phreatic event at 1050 local time which was recorded by the nearby seismic station.

Activity during 2012. Intermittent phreatic explosions continued in 2012. OVSICORI-UNA reported that seismic data revealed two eruptions on 19 and 20 February. The Red Sismologica Nacional (RSN) also reported two more small eruptions on 23 February when explosions were heard in Guachipelin (11 km SW) between 0400 and 0430, although these events were not recorded seismically. A somewhat larger eruption occurred at 1400 on 14 April 2012, which was clearly recorded by the nearby seismic equipment, visually observed, and also heard as an explosion that generated a column of vapor and sediments visible from Buenos Aires and Aguas Claras de Upala, ~7 km N of the crater (figure 18). A seismic event on 9 May that was not associated with any unusual surface activity was thought by OVSICORI-UNA scientists to be possibly associated with fluid movement inside the volcano (figure 18).

Figure 18. (left) Seismograph of the 14 April 2012 phreatic eruption of Rincon de la Vieja; (center) photo of the vapor-rich volcanic plume generated by the phreatic eruption as seen from Aguas Claras de Upala on 14 April 2012 (Photo: Yanela Zamora Ugarte); (right) monochromatic tremor not associated with unusual surface activity recorded 9 May 2012: seismograph on top, spectrogram in the middle and spectrum of frequencies at the bottom. Courtesy of OVSICORI-UNA (Monthly Bulletin, April 2012).

In October 2012, seismicity was dominated by tremors of very low amplitude and low frequency. One small phreatic eruption was recorded on 17 October 2012.

There were no significant changes in the physical or chemical parameters of the lake in 2012. In March, the temperature was 29°C and the pH was 0.42, and in December the temperature was 33°C and the pH was 0.37. Molten sulfur was observed floating on the surface during both visits. The temperatures of the fumaroles on the N flank of the volcano remained around 90-91°C, and the fumaroles SW of the crater were measured at 96-100°C during 2012.

Activity during 2013. OVSICORI-UNA received reports at 0530 on 26 February 2013 from police in Buenos Aires de Upala of pulsing white plumes rising from the active crater about every four minutes. The seismic records showed no signals associated with a phreatic eruption or sudden gas output, and no other reports from local communities were received. Cloud cover prevented thermal imaging during an overflight the next day; however, clear views of the N and S flanks and areas SW showed no evidence of ashfall, and thus OVSICORI-UNA scientists concluded that no eruptive activity had occurred. A plume from the crater was visible on a still day on 20 March from Buenos Aires de Upala (7.5 km N). In April fumarole temperatures were measured at over 130°C, and highly active convection cells with regular pulses of heat about every three minutes were observed in the lake. The lake pH was 0.1, and temperature measured with the thermal camera was 36°C.

In May 2013, Observatorio Sismológico y Vulcanológico Arenal-Miravalles del Instituto Costarricense de Electricidad (OSIVAM-ICE) reported an increase in seismic activity, characterized by an increase in the amplitude of low-frequency hybrid and VLF signals (

Figure 19. Hybrid-type seismic events recorded on 22 May 2013 at Rincon de la Vieja. Courtesy of OVSICORI-UNA (Monthly Bulletin, May 2013).

Activity during 2014. In April 2014, for the first time, scientists were able to record measurements of SO₂ emissions at Rincon de la Vieja. Values of around 70 metric tons per day were measured using a FlySpec V3 system. Seismicity remained low until September when several tremor episodes and low-frequency earthquakes occurred. OVSICORI-UNA reported that at 1237 on 17 September 2014 a seismic signal indicating a phreatic explosion was detected by a station about 5 km S of the volcano. A second explosion, detected at 2048 and lasting three minutes, was of a larger magnitude and a longer duration. Additional explosions were also detected at 1825 on 19 September and at 0304, 0439, and 0634 on 20 September. Residents on the N flank who heard the event on 19 September and saw the explosion at 0634 on 20 September reported that ejecta did not reach beyond the crater. An overflight on 20 September revealed that the temperature of the lake water was about 45°C, an increase from about 30°C measured in April. In addition, significant increases in acidity, electrical conductivity, and sulfate/fluoride and sulfate/chloride ratios were observed in September. Extensive low-frequency earthquakes and tremors (lasting for six hours) were again reported on 27 September.

On 24 October a small phreatic eruption was recorded, but otherwise seismic activity remained low. Gas measurements in early October indicated a significant decline in the CO₂/SO₂ and H₂S/SO₂ ratios of gases as compared with measurements made prior to the eruptions of September 2014. On 9 November a neighborhood north of the volcano observed a diffuse plume of steam coming from the crater, but there was no associated seismic signal, and thus OVSICORI-UNA scientists concluded that it was not a phreatic explosion. By November the fumaroles on the inner W wall of the crater were much quieter, and the lake was a milky turquoise color at a very high level due to significant precipitation (figure 20).

Figure 20. Rincon de la Vieja on 6 November 2014. Note the high water level of the lake and low-temperature fumaroles in the inner W wall of the active crater. The milky turquoise color indicates that the underwater fumarolic activity was quiet. Photo credit: Volcano National Park Rincon de la Vieja. Courtesy of OVSICORI-UNA (Monthly Bulletin, November 2014).

Activity during 2015. In May and June 2015 OVSICORI-UNA and OSIVAM-ICE installed new dataloggers and temperature sensors at two hot springs located approximately 3.2 km NE (the "Land of Senses Sanctuary") and 7.5 km N (the Blue River Sanctuary) of the active crater with the intent of monitoring changes in the water temperature and chemistry at the hot springs. At 2001 on 16 June and 1729 on 18 June, phreatic explosions that were preceded by LP and "tornillo" type seismic signals were recorded, but otherwise seismic activity was low during the month. Another small phreatic eruption that was barely distinguishable from ambient noise was recorded by OSIVAM-ICE and OVSICORI-UNA seismographs on 21 August at 0313 local time and lasted for about 80 seconds.

Activity increased substantially in October 2015, when at least ten small to moderate phreatic eruptions were recorded. On 1 October a long-period (LP) signal was recorded with a duration of 180 seconds possibly associated with a moderate exhalation of gas or vapor and a small phreatic eruption in the active crater. During a visit on 8 October, scientists observed fresh sediments and altered rock fragments around the eastern edge of the crater likely from a moderate eruption that occurred at 0624 on 4 October. Vigorous convection cells were also observed in the milky-gray lake. The most significant seismic activity occurred on 16 October and lasted for seven hours and was probably accompanied by a moderate phreatic eruption. In an aerial view on 17 October freshly ejected material was observed on the N flank (figure 21). The biggest eruption was recorded on 21 October. The largest eruptions ejected materials from the lake bottom towards the north side of the crater. Heavy rains washed these materials down river drainages. Seismicity remained low in November 2015, and no further phreatic eruptions were reported in 2015.

Figure 21. Aerial view of Rincon de la Vieja from the E on 17 October 2015 at 0900 local time. The lake is milky colored, with strong convection cells of sulfur, sediments, and gases rising from the surface. Note sediments and other materials deposited on the N flank (right side) from phreatic eruptions between 4 and 16 October 2015. Photo credit: Federico Chavarria Kopper. Courtesy of OVSICORI-UNA (October 2015 Monthly Bulletin).

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); Observatorio Sismológico y Vulcanológico Arenal-Miravalles del Instituto Costarricense de Electricidad (OSIVAM-ICE), Sección de Sismología, Vulcanología y Exploración Geofísica, Escuela Centroamericana de Geología Apdo. 214-2060, San Pedro, Costa Rica (URL: http://rsn.ucr.ac.cr/actividad-volcanica/reportes-volcanicos); Red Sismologica Nacional (RSN), Sección de Sismología, Vulcanología y Exploración Geofísica, Escuela Centroamericana de Geología, Apdo. 214-2060, San Pedro, Costa Rica (URL: http://rsn.ucr.ac.cr/); Insidecostarica.com (URL: http://www.insidecostarica.com/dailynews/2012/february/24/costarica12022404.htm).

August 2017 (BGVN 42:08) Cite this Report

Phreatic explosions disperse material up to 2 km from the active crater in March 2016 and June 2017

The active crater at Costa Rica's Rincón de la Vieja, which contains a 500-m-wide acid lake, has been the site of numerous historic eruptions at this large volcanic complex. Intermittent phreatic explosions since 2011 have dispersed volcanic debris from the crater lake within a few kilometers of the crater rim and into the surrounding streams a number of times. The most recent previous activity included explosions in September and October 2014, and phreatic eruptions on June, August and October 2015 (BGVN 41:01); this report discusses activity during 2016 and through July 2017. Information comes from the Observatorio Vulcanológico Sismológica de Costa Rica-Universidad Nacional (OVSICORI-UNA) and the Observatorio Sismológico y Vulcanológico de Arenal-Miravalles (OSIVAM-ICE). The OVISAM-ICE reports are published through the Red Sismológica Nacional (RSN), the National Seismological Network. Ejected material is described in the original reports in various ways that appear to be interchangeable rather than signifying actual content differences, so those distinctions are not reflected below unless ash was specified.

The first evidence of a new episode of phreatic explosions was noted during a site visit on 15 February 2016. Numerous explosions during March spread material as far as 2 km from the crater rim. After an explosion on 1 May 2016 there were no further reports until 23 May 2017, when a series of intermittent explosions again ejected material onto the N and NW flanks and sent plumes of steam-and-gas as high as 2 km above the crater rim. The last reported explosion was on 5 July 2017.

Activity decreased at the end of 2015 after the phreatic explosions of 16-21 October. The number of seismic events increased again during February and March 2016. OVSICORI-UNA scientists observed the first evidence of a new episode of phreatic explosions during a field visit on 15 February 2016 when they noted deposits about 20 m from the crater rim. By the end of March, the RSN had reported 25 explosions. Three of the largest explosions occurred on 9 February, 9 March, and 18 March. They were characterized by episodes of tremor in pulses that usually lasted about five minutes prior to the phreatic explosion, and then changed to continuous tremor for several hours afterwards.

OSIVAM-ICE scientists reported photographic evidence of deposits from a 2 March explosion that covered a wide area on the N flank of the active crater (figure 22). They visited on 3 March 2016 and noted fresh deposits from the phreatic explosions about 200 m W of the crater rim (figure 23). They also witnessed three explosions during the afternoon, the longest lasting for 65 seconds.

Figure 22. Deposits of material ejected from the crater lake on the N edge of Rincón de la Vieja associated with an eruptive event that occurred on 2 March 2016 at 1747 local time. Photo from Fernando Madrigal's Sensoria site, courtesy of RSN (Resumen de la actividad sísmica y eruptive del volcán Rincón de la Vieja (Costa Rica) 01 de octubre del 2015 al 15 de marzo del 2016).

Figure 23. Deposit of material from the crater lake at Rincón de la Vieja on 3 March 2016, located about 200 m W of the crater rim. Photo by OSIVAM-ICE scientists, courtesy of RSN (Resumen de la actividad sísmica y eruptive del volcán Rincón de la Vieja (Costa Rica) 01 de octubre del 2015 al 15 de marzo del 2016).

Scientists from OVSICORI-UNA conducted additional site visits during 8 and 10-11 March 2016. On 8 March fresh ash was found about 120 m from the crater rim (figure 24), and a temperature of 55°C was measured remotely for the convection cell in the lake. Based on photographs taken by nearby residents, OVSICORI-UNA scientists estimated that the ash and steam plumes produced by the 9 and 10 March explosions rose 700 and 850 m, respectively, above the crater. Local residents reported to The Tico Times that ash fell on the roofs of their homes within an area up to 6 km around the volcano after the explosion on 9 March, mostly in communities N of the crater (Upala and Buenos Aires).

Figure 24. The N rim of the active crater at Rincón de la Vieja on 8 March 2016 is marked with the outline (white dashes) showing the extent of material ejected during recent explosions. The arrow at the top shows the dominant wind direction. Inset on left shows riverbed deposits of recent material on 8 March, and the right inset images show the plumes from the 9 (upper) and 10 (lower) March explosions. Right inset photos by Jorge Viales, courtesy of OVSICORI (Erupciones del volcán Rincón de la Vieja: Observaciones de Campo).

The character of the deposits changed between February and March 2016, according to a report by OVSICORI scientists. The samples collected in February were rich in elemental sulfur, abundant in the crater lake and in the near-surface sediments. Studies of the March samples showed the presence of clasts of altered rocks, hydrothermal minerals, and elemental sulfur as well as 3-10% fresh glass.

During their summit visit on 10 and 11 March 2016, OVSICORI scientists noted a coating of white sediment, up to 5 mm thick in some places, covering the ground and the vegetation in a 400m-wide area to the SSW of the active crater (figure 25). Deposits extended as far as 2 km away, and coated the flanks of both the active crater and the nearby Von Seeback crater (figure 26).

Figure 25. Material from phreatic explosions cover a Copey shrub at Rincón de la Vieja on 10 March 2016. The plant was located 1.5 km SSW from the active crater. Photo by E. Duarte, courtesy of OVSICORI-UNA (Visita al Volcán Rincón de la Vieja: Mapeo de Efecto y Características de Erupciones Freáticas Recientes).

Figure 26. A view to the ESE on 10 March 2016 from the flank of the Von Seeback crater towards the active crater showing the coating of white sediments from the recent phreatic explosions at Rincón de la Vieja. The arrow points roughly NW showing the direction of sediment dispersal. Material was sampled at site 4 (white circle). Photo by E. Duarte, courtesy of OVSICORI-UNA (Visita al Volcán Rincón de la Vieja: Mapeo de Efecto y Características de Erupciones Freáticas Recientes).

A 15 March explosion generated a 700-m-high plume of water vapor and gas, according to an announcement from OVSICORI-UNA. They also reported an explosion on 1 May 2016 detected for 11 minutes by the seismic network. No further reports were made until May 2017.

A small lahar traveled down the N flank of the crater after an explosion on 23 May 2017. Explosions on 11 and 12 June were recorded seismically, but cloudy weather obscured visual observations. The Washington VAAC, however, noted a hotspot in the infrared satellite data on 11 June 2017 about 30 minutes before the explosion was reported. A diffuse steam plume was observed from Dos Rios de Upala rising about 50 m above the summit on 15 June, and a small phreatic explosion was recorded on 18 June 2017. A larger explosion on 23 June sent a plume 1-2 km above the summit, and ejected material to the W and NW onto the upper N flank toward the Von Seebach crater 2 km to the W. Small phreatic explosions on 5 July ejected material that did not rise above the crater rim.

Information Contacts: Observatorio Vulcanológico Sismológica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); Observatorio Sismológico y Vulcanológico Arenal-Miravalles del Instituto Costarricense de Electricidad (OSIVAM-ICE), Sección de Sismología, Vulcanología y Exploración Geofísica, Escuela Centroamericana de Geología, Apdo. 214-2060, San Pedro, Costa Rica (URL: http://rsn.ucr.ac.cr/); The Tico Times (URL: http://www.ticotimes.net/2016/03/10/costa-rica-rincon-de-la-vieja-volcano-vapor-ash-explosions).

March 2018 (BGVN 43:03) Cite this Report

Phreatic explosions during 29 September-22 October 2017

During the first half of 2017, phreatic explosions at Rincón de la Vieja occurred on 23 May, 11-12 June (however, clouds obscured visible observations), 18 and 23 June, and 5 July (BGVN 42:08). This report describes activity from 6 July through December 2017. Information comes from the Observatorio Vulcanológico Sismológica de Costa Rica-Universidad Nacional (OVSICORI-UNA).

After a small phreatic explosion on 5 July 2017, there were no further reports of any explosions until 29 September when OVSICORI-UNA reported that at 0848 a small phreatic explosion produced a plume that rose 1 km above the crater rim (figure 27); material also flowed down the S flank.

Figure 27. Webcam image of a phreatic explosion at Rincón de la Vieja on 29 September 2017. Courtesy of OVSICORI-UNA (color adjusted).

According to OVSICORI-UNA, events on 3 October at 0848 and 1445 generated plumes that rose 700 m and 1,500 m, respectively. OVSICORI-UNA also reported that on 9 October at 1048, a small explosion produced a plume that rose 700 m above the crater rim. According to news reports (The Costa Rica Star and CRHoy.com) quoting OVSICORI-UNA, an explosion on 22 October at 0640 generated a steam-and-gas plume that rose about 1 km above the crater. There were no further reports of an explosion through the end of December.

Information Contacts: Observatorio Vulcanológico Sismológica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/, https://www.facebook.com/OVSICORI/); CRHoy.com (URL: http://www.crhoy.com/); The Costa Rica Star (URL: https://news.co.cr/).

September 2018 (BGVN 43:09) Cite this Report

Intermittent weak phreatic explosions during January-March and July-August 2018

The Rincón de la Vieja volcano complex has generated intermittent phreatic explosions since 2011; during 2017, weak phreatic explosions occurred during May, June, July, September, and October (BGVN 42:08 and 43:03). This activity continued through August 2018. The volcano is monitored by the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA).

According to OVSICORI-UNA, at 1758 on 9 January 2018, an explosion produced a plume that rose 1 km above the crater rim. On 12 January, OVSICORI-UNA reported some small phreatic explosions. The webcam detected weak explosions again in mid-February. Another weak explosion on 22 February confirmed the presence of a crater lake.

During the first week of March OVSICORI-UNA reported weak phreatic explosions of low amplitude that were only be detected by the webcam (figure 28), and not by seismic instruments. During the week of 5-11 March there were 2-4 weak phreatic explosions occurred per day, along with strong tremor on the 10th. Small eruptions were seen on unspecified days the week of 12-18 March.

Figure 28. Webcam image of a phreatic explosion at Rincon de la Vieja on 3 March 2018. Courtesy of OVSICORI-UNA.

No phreatic activity was reported during the second half of March through June, though on 20 May a seismic swarm of about 30 earthquakes was recorded. After a tremor on 3 July, a possible weak phreatic explosion occurred on 4 July at 0044, followed by a pulse of tremor. On 28 July, at 1828, a small explosion followed by tremor was recorded.

On 3 August OVSICORI-UNA reported that two weak explosions occurred at dawn. On 14 August, another weak explosion began at 1828 and lasted three minutes. Foggy conditions prevented webcam views and an estimate of a plume height. Other weak explosions were recorded on 17 August at 1407 and 2015.

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/).

April 2019 (BGVN 44:04) Cite this Report

Occasional weak phreatic explosions continue through February 2019

Intermittent small phreatic explosions from the acid lake of Rincón de la Vieja's active crater has most recently occurred since 2011 (BGVN 42:08, 43:03, and 43:09). This activity continued through at least February 2019. The volcano is monitored by the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), and the information below comes from its weekly bulletins between 18 August 2018 and 28 February 2019. Weather conditions often prevented webcam views and estimates of plume heights. The volcano was in Activity Level 3 throughout the reporting period (volcano erupting, steady state).

According to OVSICORI-UNA, two distinct, 2-minute-long explosions occurred on 31 August 2018 beginning at 0434 and 1305. Several hours after the eruption tremor became continuous but low-frequency long-period (LP) earthquakes ceased. OVSICORI-UNA reported a gas emission late on 7 September. An unconfirmed small phreatic explosion occurred on 11 September at 0634, and another on 17 September at 1014. The seismic record showed continuous background tremor and very sporadic LP earthquakes.

Intermittent background tremor was recorded during the first half of October, along with a few emissions and phreatic explosions. Deformation measurements during October showed a contraction between the N and S of the volcano, with subsidence. On 17 October there was another phreatic explosion, and thereafter tremor disappeared and seismicity decreased. On 23 and 27 October seismic stations signaled additional possible phreatic explosions.

OVSICORI-UNA reported that a series of explosions began at 1945 on 4 November and consisted of at least three 2-minute-long episodes. The next day at 1511 a plume of water vapor and diffuse gas, recorded by a webcam and visible to residents to the N, rose about 100 m above the crater rim and drifted W. On 9 November a 2-minute-long explosion began at 1703. Another explosion on 27 November at 0237 produced a plume of water vapor and gas that rose 600 m above the crater rim and drifted SW. A short 1-minute explosion began at 1054 on 3 December.

Based on OVSICORI-UNA weekly bulletins, activity remained stable in January 2019 with small-amplitude phreatic explosions on 11, 12, and 14 January. More energetic phreatomagmatic explosions on 17 and 20 January produced lahars. Several small-amplitude explosions were detected at the end of the month. During January, a few LPs, no VTs, and intermittent tremor were recorded.

OVSICORI-UNA reported that two small-scale explosions occurred on 1 February, along with possible events at 1906 and 1950 on 5 February and at 0120 on 6 February. An event at 0000 on 6 February was also recorded; the report noted that poor weather conditions prevented visual observations of the crater. On 16 and 17 February strong degassing was observed. No LPs were recorded, but two significant VTs were detected on 17 and 22 February near or under the crater.

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/).

August 2019 (BGVN 44:08) Cite this Report

Occasional weak phreatic explosions during March-July 2019

The acid lake of Rincón de la Vieja's active crater has generated intermittent weak phreatic explosions regularly since 2011, continuing during the past year through at least August 2019. The volcano is monitored by the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), and the information below comes from its weekly bulletins between 4 March and 2 September 2019. Clouds often prevented webcam and satellite views. The current report describes activity from March through July 2019.

OVSICORI-UNA reported that weak events occurred on 19 March at 1851 and on 29 March 2019 at 2043. A two-minute-long phreatic explosion on 1 April at 0802 produced a plume that rose 600 m above the crater rim. Continuous emissions were visible during 3-4 April, rising 200 m above the crater rim. On 3 April, at 1437, a small explosion was detected. An explosion on 10 April at 0617 produced a gas-and-steam plume that rose 1 km above the crater rim and drifted SE. On 12 April at 0643, a plume rose 500 m. Another event took place at 0700 on 13 April, although poor weather conditions prevented visual observations. On 14 April, OVSICORI-UNA noted that aerial photographs showed a milky-gray acid lake at a relatively low water level with convection cells of several tens meters of diameter in the center and eastern parts of the lake.

According to an OVSICORI-UNA bulletin, a small phreatic explosion occurred on 1 May. Another explosion on 11 May at 0720 produced a white gas-and-steam plume that rose 600 m above the crater rim. Phreatic explosions were recorded on 14 May at 1703 and on 17 May at 0357, though dense fog prevented visual confirmation of both events with webcams. On 15 May a local observer noted a diffuse plume of steam and gas, material rising from the crater, and photographed milky-gray deposits on the N part of the crater rim ejected from the event the day before. A major explosion occurred on 24 May.

OVSICORI-UNA recorded a significant phreatic 10-minute-long explosion that began on 11 June at 0343, but plumes were not visible due to weather conditions. No further phreatic events were reported in July.

Seismic activity was very low during the reporting period, and there was no significant deformation. Short tremors were frequent toward the end of April, but were only periodic in May and June; tremor almost disappeared in July. A few long-period earthquakes were recorded, and volcano-tectonic earthquakes were even less frequent.

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/, https://www.facebook.com/OVSICORI/).

April 2020 (BGVN 45:04) Cite this Report

Weak phreatic explosions during August 2019-March 2020; ash and lahars reported in late January

Rincón de la Vieja is a remote volcanic complex in Costa Rica containing an acid lake that has regularly generated weak phreatic explosions since 2011 (BGVN 44:08). The most recent eruptive period occurred during late March-early June 2019, primarily consisting of small phreatic explosions, minor deposits on the N crater rim, and gas-and-steam emissions. The report period of August 2019-March 2020 was characterized by similar activity, including small phreatic explosions, gas-and-steam plumes, ash and lake sediment ejecta, and volcanic tremors. The most significant activity during this time occurred on 30 January, where a phreatic explosion ejected ash and lake sediment above the crater rim, resulting in a pyroclastic flow which gradually turned into a lahar. Information for this reporting period of August 2019-March 2020 comes from the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA) using weekly bulletins.

According to OVSICORI-UNA, a small hydrothermal eruption was recorded on 1 August 2019. The seismicity was low with a few long period (LP) earthquakes around 1 August and intermittent background tremor. No explosions or emissions were reported through 11 September; seismicity remained low with an occasional LP earthquake and discontinuous tremor. The summit’s extension that has been recorded since the beginning of June stopped, and no significant deformation was observed in August.

Starting again in September 2019 and continuing intermittently through the reporting period, some deformation was observed at the base of the volcano as well as near the summit, according to OVSICORI-UNA. On 12 September an eruption occurred that was followed by volcanic tremors that continued through 15 September. In addition to these tremors, vigorous sustained gas-and-steam plumes were observed. The 16 September weekly bulletin did not describe any ejecta produced as a result of this event.

During 1-3 October small phreatic eruptions were accompanied by volcanic tremors that had decreased by 5 October. In November, volcanism and seismicity were relatively low and stable; few LP earthquakes were reported. This period of low activity remained through December. At the end of November, horizontal extension was observed at the summit, which continued through the first half of January.

Small phreatic eruptions were recorded on 2, 28, and 29 January 2020, with an increase in seismicity occurring on 27 January. On 30 January at 1213 a phreatic explosion produced a gas column that rose 1,500-2,000 m above the crater, with ash and lake sediment ejected up to 100 m above the crater. A news article posted by the Universidad de Costa Rica (UCR) noted that this explosion generated pyroclastic flows that traveled down the N flank for more than 2 km from the crater. As the pyroclastic flows moved through tributary channels, lahars were generated in the Pénjamo river, Zanjonuda gorge, and Azufrosa, traveling N for 4-10 km and passing through Buenos Aires de Upala (figure 29). Seismicity after this event decreased, though there were still some intermittent tremors.

Figure 29. Photo of a lahar generated from the 30 January 2020 eruption at Rincon de la Vieja. Photo taken by Mauricio Gutiérrez, courtesy of UCR.

On 17, 24, and 25 February and 11, 17, 19, 21, and 23 March, small phreatic eruptions were detected, according to OVSICORI-UNA. Geodetic measurements observed deformation consisting of horizontal extension and inflation near the summit in February-March. By the week of 30 March, the weekly bulletin reported 2-3 small eruptions accompanied by volcanic tremors occurred daily during most days of the week. None of these eruptions produced solid ejecta, pyroclastic flows, or lahars, according to the weekly OVSICORI-UNA bulletins during February-March 2020.

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/, https://www.facebook.com/OVSICORI/); Luis Enrique Brenes Portuguéz, University of Costa Rica, Ciudad Universitaria Rodrigo Facio Brenes, San José, San Pedro, Costa Rica (URL: https://www.ucr.ac.cr/noticias/2020/01/30/actividad-del-volcan-rincon-de-la-vieja-es-normal-segun-experto.html).

October 2020 (BGVN 45:10) Cite this Report

Frequent small phreatic explosions with intermittent ash plumes during April-September 2020

Rincón de la Vieja is a remote volcanic complex in Costa Rica that contains an acid lake. Frequent weak phreatic explosions have occurred since 2011 (BGVN 44:08). The most recent eruption period began in January 2020, which consisted of small phreatic explosions, gas-and-steam plumes, pyroclastic flows, and lahars (BGVN 45:04). This reporting period covers April through September 2020, with activity characterized by continued small phreatic explosions, three lahars, frequent gas-and-steam plumes, and ash plumes. The primary source of information for this report is the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA) using weekly bulletins and the Washington Volcanic Ash Advisory Center (VAAC).

Small, frequent, phreatic explosions were common at Rincón de la Vieja during this reporting period. One to several eruptions were reported on at least 16 days in April, 15 days in May, 8 days in June, 10 days in July, 18 days in August, and 13 days in September (table 5). Intermittent ash plumes accompanied these eruptions, rising 100-3,000 m above the crater and drifting W, NW, and SW during May and N during June. Occasional gas-and-steam plumes were also observed rising 50-2,000 m above the crater rim.

Table 5. Monthly summary of activity at Rincón de la Vieja during April through September 2020. Courtesy of OVSICORI-UNA.

During April small explosions were detected almost daily, some of which generated ash plumes that rose 200-1,000 m above the crater and gas-and-steam emissions that rose 50-1,500 m above the crater. On 4 April an eruption at 0824 produced an ash plume that rose 1 km above the crater rim. A small hydrothermal explosion at 0033 on 11 April, recorded by the webcam in Sensoria (4 km N), ejected water and sediment onto the upper flanks. On 15 April a phreatic eruption at 0306 resulted in lahars in the Pénjamo, Azufrada, and Azul rivers, according to local residents. Several small explosions were detected during the morning of 19 April; the largest phreatic eruption ejected water and sediment 300 m above the crater rim and onto the flanks at 1014, generated a lahar, and sent a gas-and-steam plume 1.5 km above the crater (figure 30). On 24 April five events were recorded by the seismic network during the morning, most of which produced gas-and-steam plumes that rose 300-500 m above the crater. The largest event on this day occurred at 1020, ejecting water and solid material 300 m above the crater accompanied by a gas-and-steam plume rising up to 1 km.

Figure 30. Webcam image of small hydrothermal eruptions at Rincón de la Vieja on 19 April 2020. Image taken by the webcam in Dos Ríos de Upala; courtesy of OVSICORI-UNA.

Similar frequent phreatic activity continued in May, with ash plumes rising 200-1,500 m above the crater, drifting W, NW, and SW, and gas-and-steam plumes rising up to 2 km. On 5 May an eruption at 1317 produced a gas-and-steam plume 200 m above the crater and a Washington VAAC advisory reported that an ash plume rose to 2.1 km altitude, drifting W. An event at 1925 on 9 May generated a gas-and-steam plume that rose almost 2 km. An explosion at 1128 on 15 May resulted in a gas-and-steam plume that rose 1 km above the crater rim, accompanied by a gray, sediment-laden plume that rose 400 m. On 21 May a small ash eruption at 0537 sent a plume 1 km above the crater (figure 31). According to a Washington VAAC advisory, an ash plume rose 3 km altitude, drifting NW on 22 May. During the early evening on 25 May an hour-long sequence of more than 70 eruptions and emissions, according to OVSICORI-UNA, produced low gas-and-steam plumes and tephra; at 1738, some ejecta was observed above the crater rim. The next day, on 26 May, up to 52 eruptive events were observed. An eruption at 2005 was not visible due to weather conditions; however, it resulted in a minor amount of ashfall up to 17 km W and NW, which included Los Angeles of Quebrada Grande and Liberia. A phreatic explosion at 1521 produced a gray plume that rose 1.5 km above the crater (figure 31). An eruption at 1524 on 28 May sent an ash plume 3 km above the crater that drifted W, followed by at least three smaller eruptions at 1823, 1841, and 1843. OVSICORI-UNA reported that volcanism began to decrease in frequency on 28-29 May. Sulfur dioxide emissions ranged between 100 and 400 tons per day during 28 May to 15 June.

Figure 31. Webcam images of gray gas-and-steam and ash emissions at Rincón de la Vieja on 21 (left), and 27 (right) May 2020. Both images taken by the webcam in Dos Ríos de Upala and Sensoria; courtesy of OVSICORI-UNA.

There were eight days with eruptions in June, though some days had multiple small events; phreatic eruptions reported on 1-2, 13, 16-17, 19-20, and 23 June generated plumes 1-2 km above the crater (figure 32). During 2-8 June SO₂ emissions were 150-350 tons per day; more than 120 eruptions were recorded during the preceding weekend. Ashfall was observed N of the crater on 4 June. During 9-15 June the SO₂ emissions increased slightly to 100-400 tons per day. During 16-17 June several small eruptive events were detected, the largest of which occurred at 1635 on 17 June, producing an ash plume that rose 1 km above the crater.

Figure 32. Webcam images of gray gas-and-steam and ash plumes rising from Rincón de la Vieja on 1 (top left), 2 (top right), 7 (bottom left), and 13 (bottom right) June 2020. The ash plume on 1 June rose between 1.5 and 2 km above the crater. The ash plume on 13 June rose 1 km above the crater. Courtesy of OVSICORI-UNA.

Explosive hydrothermal activity was lower in June-September compared to January-May 2020, according to OVSICORI-UNA. Sporadic small phreatic explosions and earthquakes were registered during 22-25 and 29 July-3 August, though no lahars were reported. On 25 July an eruptive event at 0153 produced an ash plume that rose 1 km above the crater. Similar activity continued into August. On 5 and 6 August phreatic explosions were recorded at 0546 and 0035, respectively, the latter of which generated a plume that rose 500 m above the crater. These events continued to occur on 10, 16, 19-20, 22-25, 27-28, and 30-31 August, generating plumes that rose 500 m to 1 km above the crater.

On 3 September geologists observed that the acid lake in the main crater had a low water level and exhibited strong gas emissions; vigorous fumaroles were observed on the inner W wall of the crater, measuring 120°C. Gas-and-steam emissions continued to be detected during September, occasionally accompanied by phreatic eruptions. On 7 September an eruption at 0750 produced an ash plume that rose 50 m above the crater while the accompanying gas-and-steam plume rose 500 m. Several low-energy phreatic explosions occurred during 8-17, 20, and 22-28 September, characterized primarily by gas-and-steam emissions. An eruption on 16 September ejected material from the crater and generated a small lahar. Sulfur dioxide emissions were 100 tons per day during 16-21 September. On 17 September an eruption at 0632 produced an ash plume that rose 700 m above the crater (figure 33). A relatively large eruptive event at 1053 on 22 September ejected material out of the crater and into N-flank drainages.

Figure 33. Webcam image of an eruption plume rising above Rincón de la Vieja on 17 September 2020. Courtesy of OVSICORI-UNA.

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/, https://www.facebook.com/OVSICORI/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html).

March 2021 (BGVN 46:03) Cite this Report

Frequent small phreatic explosions through 13 December 2020

Recent activity at Rincón de la Vieja has been dominated by frequent weak phreatic explosions, with an occasional ash plume, along with gas-and-steam emissions. Sporadic lahars have also been recently reported (BGVN 45:10). The volcano has a hot, churning, acid lake in its main crater. The current report describes activity during October 2020-February 2021, a continuation of the most recent eruptive period that began in January 2020. The primary information source for this report is weekly bulletin from the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA).

According to OVSICORI-UNA, small but frequent hydrothermal explosions continued in October through mid-December 2020, although less energetic than during previous months (figure 34). During the first half of October there were 1-2 daily small explosions. Plumes often rose 500-800 m above the crater rim, but on 1 and 6 October they rose 1 km. Then the number briefly increased to 5-7 small daily explosions before decreasing during the latter part of October; one explosion seen in webcam images on 24 October sent a plume to 1 km above the crater (figure 35).

Figure 34. Graph showing the number of daily eruptions at Rincón de la Vieja during 2020. Following frequent phreatic explosions during April-June, weak intermittent explosions were detected again starting in late July and continuing through December 2020. Courtesy of OVSICORI-UNA.

Figure 35. Webcam photo of Rincón de la Vieja taken on 24 October 2020 at 0808 local time. According to OVSICORI-UNA, the explosion lasted for about a minute and the resulting plume rose to 1 km above the crater. Courtesy of OVSICORI-UNA, as reported by The Nacion.

OVSICORI-UNA reported that in November small-to-moderate hydrothermal explosions increased in amplitude, but became more sporadic and by the end of the month had decreased to only one per day. An explosion at 0835 on 3 November produced a plume that rose 800 m above the crater rim. According to OVSICORI’s weekly bulletin for 23 November, there had been 1,437 explosions since the beginning of 2020. A large explosion on 13 December was the last through at least February 2021. During the week of 18 January OVSICORI changed the Alert Level from 3 to 2 due to the low level of activity.

Geodesic monitoring at the summit by GPS indicated no deformation trend in October, significant contraction in November, some extension in December, but then no significant changes through at least February 2021. Aerial observations on 13 February indicated that the crater lake was at a low water level and had sustained convection. The lake level had dropped 15-20 m since February 2020, and 5-10 m since May 2020. Gas monitoring during October 2020-February 2021 was carried out at the Ojo de Agua Santuarium (4 km N of the active crater); sulfur dioxide in the plume was not detected in significant quantities.

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/, https://www.facebook.com/OVSICORI/); The Nacion (URL: https://www.nacion.com/).

November 2021 (BGVN 46:11) Cite this Report

Powerful explosion on 28 June 2021 sends plume 5 km above crater

Activity at Rincón de la Vieja, in Costa Rica, is typically characterized by weak phreatic explosions that generate gas-and-steam emissions and plumes. This report covers activity during March-August 2021, and is based primarily on weekly bulletins from the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA) and satellites. Almost all of the reported explosions during March-August 2021 (table 6) were identified by OVSICORI-UNA as being phreatic; no explosions occurred after 13 December 2020 until 29 May 2021.

Table 6. Summary of activity at Rincón de la Vieja during March-August 2021 from weekly bulletins. Courtesy of OVSICORI-UNA.

The Alert Level was raised from 2 to 3 following the renewal of heightened activity on 29 May 2021, when two phreatic explosions were recorded. Additional minor phreatic explosions were detected on 1, 6, 15, and possibly 20 June. A more powerful explosion on 28 June generated a plume of steam, gas, and ash that rose 2 km above the crater rim and ejected material onto its flanks (figures 36 and 37). Part of the plume rose up to 5 km above the crater (7 km altitude), as measured from Turrialba volcano, more than 200 km SE. Residents in Gavilán and Dos Ríos (7 km NNW) and Bromelias (6 km NNE) reported volcanic gas odors and ashfall. Lahars descended multiple drainages on the N flank. OVSICORI-UNA officials were quoted by news organizations (The Costa Rica News and CNN) as stating that the 28 June activity was the most powerful explosion since at least 2011. Intermittent explosions continued through July and August, but weather conditions typically prevented visual observations.

Figure 36. Image of the eruption plume from the 28 June 2021 explosion at Rincón de la Vieja taken at 0542 local time. Courtesy of OVSICORI-UNA.

Figure 37. View of the ash-and-steam plume following the 28 June 2021 explosion at Rincón de la Vieja. Courtesy of OVSICORI-UNA, as published by Periódica Mensaje.

Seismicity was low during the reporting period, with occasional long-period (LP) earthquakes, tornillos, volcano tectonic events, and tremor. In March the frequency and amplitude of background tremor decreased, and remained low in April. After 24 April, short tremors of large amplitude and low frequency (below 1Hz) were recorded until early May, when they decreased in amplitude. During May and June, the number of LP earthquakes was 0-2 events per day. Before the 31 May explosion, tremor was banded (series of "tremor packets" occurring at almost regular intervals) with frequencies below 2 Hz. During the first week of June, tremor frequency was around 2.2 Hz, but decreased toward the end of the month. Tremor on 26 June had a very low amplitude and almost monochromatic frequency around 2.2 Hz. During the four days preceding the major explosion on 28 June, the number of LP earthquakes had increased from 3 to 9 daily events. Tremor had stopped by 3 August, but increased again on 30 August.

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/, https://www.facebook.com/OVSICORI/); The Costa Rica News (URL: https://thecostaricanews.com/); CNN (URL: https://www.cnn.com/); Periódica Mensaje (URL: https://www.periodicomensaje.com/).

July 2022 (BGVN 47:07) Cite this Report

Numerous weak explosions from September 2021 through June 2022

Rincón de la Vieja, a remote volcanic complex in NW Costa Rica with an acid lake, exhibits numerous weak phreatic (hydrothermal) explosions along with associated gas-and steam emissions and an occasional plume (BGVN 46:11). This report covers activity during September 2021-June 2022, and is based primarily on weekly bulletins and occasional daily reports from the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA) and satellites.

During September 2021-June 2022, the level of activity continued as before, with frequent weak phreatic explosions every month (table 7), except that no explosions were reported between 21 October and 25 December 2021. Based on OVSICORI-UNA reports, plumes rose no more than 500 m above the crater rim, with the exception of stronger explosions on 25 January, 7 April, and 6 June. Explosions produced lahars on 5 and 13 January. OVSICORI-UNA reports suggested that an explosion on 5 January may have been phreatomagmatic in nature (i.e., interaction between water and magma).

Table 7. Summary of activity at Rincón de la Vieja during September 2021-June 2022. Explosions were weak and phreatic. Courtesy of OVSICORI-UNA (compiled from weekly bulletins and occasional daily reports).

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/, https://www.facebook.com/OVSICORI/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html).

January 2023 (BGVN 48:01) Cite this Report

Continuing frequent weak phreatic explosions during July-December 2022

Rincón de la Vieja is a volcanic complex in Costa Rica with a hot, convecting acid lake that exhibits frequent weak phreatic explosions, gas-and-steam emissions, and occasional elevated sulfur dioxide levels (BGVN 45:10, 46:03, 46:11). This report covers activity during July-December 2022 and is based on weekly bulletins and occasional daily reports from the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA).

During this period, OVSICORI-UNA reported that eruptive activity continued similar to recent years, with weak phreatic explosions and gas-and-steam emissions generating plumes above the lake (table 8). The sulfur dioxide levels were generally stable at 30-50 metric tons/day (t/d) during July-October, with isolated peaks up to 600 t/d. In early November, isolated peaks of 750 t/d were measured, and thereafter levels varied widely. On 18 November the level peaked at 1,500 t/d. Toward the end of December, the level was about 250 t/d.

Table 8. Summary of activity at Rincón de la Vieja during July-December 2022. Some eruptions that failed to eject lake water and sediments outside the crater were not considered by OVSICORI-UNA as having an explosive character, so eruptive activity terminology is variable. Explosions were weak and phreatic. Courtesy of OVSICORI-UNA (compiled from weekly bulletins and occasional daily reports).

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/, https://www.facebook.com/OVSICORI/).

July 2023 (BGVN 48:07) Cite this Report

Phreatic eruptions and gas-and-steam plumes persisted during January-June 2023

Rincón de la Vieja is a remote volcanic complex in the Guanacaste Range in NW Costa Rica. It consists of an elongated, arcuate NW-SE-trending ridge constructed within the 15-km-wide Guachipelín caldera, whose rim is exposed on the south side. Eruptions dating back to the 16th century have been from the active crater located ENE of Von Seebach crater; the active crater also contains a 500-m-wide acid lake. The current eruption period began June 2021 and has more recently consisted of weak phreatic explosions (BGVN 48:01). This report covers activity during January through June 2023 using information from weekly bulletins and occasional daily reports from the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA).

Activity was relatively low during January through February 2023. An eruption was reported at 1500 on 7 January, followed by observations of gray water in the Niño River. According to OVSICORI-UNA, these observations may have indicated that some material from the eruption came out of the crater. During 0537-0547 on 8 January a series of eruptions was detected.

On 2 March at 0651 and on 4 March weak gas-and-steam emissions were recorded, rising less than 500 m above the crater. A series of small eruptions occurred during 2305 on 10 March to 0137 on 11 March with at least four pulses confirmed in webcam images at 2305, 0000, 0006, and 0137. Continuous tremor was reported on 11 March. Weak gas-and-steam plumes sometimes rose higher than 500 m above the crater, but no ash was visible. Two eruptions were detected at 0529 and 0549 on 14 March, which generated a gas-and-steam plume that rose 2 km above the crater and drifted W and NW (figure 38). On 19 March at 1051 small gas-and-steam emissions rose less than 500 m above the crater.

Figure 38. Photo of a white gas-and-steam plume rising 2 km above the crater rim of Rincón de la Vieja on 14 March 2023. Courtesy of OVSICORI-UNA.

Tremor was reported starting around 2230 on 10 April. A small phreatic eruption was reported on 11 April; several pulses occurred between 0653 and 0700, the largest of which was recorded at 0656. A resulting gas-and-steam plume rose 500 m above the crater. Another gas-and-steam plume was recorded at 0743; tremor decreased in amplitude after 0900. Two small eruptions were recorded during the night and early morning during 11-12 April; the strongest of which occurred at 0450 and generated a gas-and-steam plume that rose 500 m above the crater and drifted SW. Two more eruptions were detected at 1920 on 12 April and at 0632 on 13 April; gas-and-steam plumes accompanied these events, but no ash was observed. On 14 April at 0455 low-frequency earthquakes were detected and an eruption was reported at 0935 that continued until after 2100; the resulting gas-and-steam plume rose 2 km above the crater and drifted SW. Another eruption was detected at 1826 on 14 April. On 16 April a small eruption was recorded at 0647 and lasted approximately one minute and generated a small lahar in the upper part of the basin. An accompanying gas-and-steam plume rose 2 km above the summit (figure 39). Another eruptive event occurred at 0957 and produced a gas-and-steam plume that rose 500 m above the summit. On 19 April several eruptions generating gas-and-steam plumes were reported, which rose as high as 1.5 km above the crater and drifting W. On 21 April at 1550 a strong eruption was reported. The accompanying gas-and-steam plume rose 4-5 km above the crater and drifted SW (figure 40). Lahars were observed on the N flank. Continuous degassing was reported after this eruption event, as well as strong tremor. On 21 April at 1904 a small eruption was detected in infrasound and seismic records. Near-continuous degassing persisted in the crater. On 22 April at 1904 a small eruption was detected in infrasound and seismic records and near-continuous degassing was observed in the crater. Three gas-and-steam emissions were detected on 26 April at 0453, 0523, and 0545. An eruption event at 0528 on 27 April generated a gas-and-steam plume that rose 800 m above the crater. A small eruption at 1030 on 30 April produced a gas-and-steam plume that rose 500 m above the crater accompanied by a near-continuous tremor. Several volcano-tectonic (VT) earthquakes were also detected both distally and proximally to the crater.

Figure 39. Photo of an eruption at Rincón de la Vieja at 0647 on 16 April 2023 that generated a white gas-and-steam plume and a small lahar in the upper part of the basin. Courtesy of OVSICORI-UNA.

Figure 40. Photo of the strong eruption at 1550 on 21 April 2023. The accompanying gas-and-steam plume rose 4-5 km above the crater and drifted SW. Courtesy of OVSICORI-UNA.

A moderate phreatic eruption was recorded at 1306 on 1 May that generated a gas-and-steam plume that rose 1 km above the crater. Residents to the N, seismographs, infrasound systems, and webcams all recorded the event. Seismographs recorded intermittent volcanic tremor signal and several VT-type earthquakes were recorded both distally and proximally to the crater. A small phreatic eruption was recorded at 1847 on 5 May based on data from seismographs and the infrasound system. Intermittent volcanic tremor and several low-frequency volcanic earthquakes were also detected. A short-lived explosive and energetic eruption was detected at 0258 on 14 May that produced lahars that flowed down the Pénjamo River, the Azul River, and the Azufrado River. Volcanic tremor decreased substantially on 13 May, though continuous degassing was reported. On 17 May four small phreatic eruptions were recorded, one of which occurred at 1255 that generated a gas-and-steam plume that rose 700 m above the crater. Gas-and-steam emissions persisted from the Main Crater and intermittent volcanic tremor were recorded. A gas-and-steam plume was detected in webcam, infrasound, and seismic data at 1349 on 22 May that rose 1 km above the crater. A gas-and-steam plume rose 2.5 km above the crater at 1815 on 24 May possibly containing some material from the lake.

Two moderate phreatic eruptions were detected at 1815 and 1830 on 24 May that produced gas-and-steam plumes and rose 2.5 km above the crater. The eruptions were detected by residents to the N, seismographs, the infrasound system, and webcams. Intermittent volcanic tremor was also recorded. Aerial photographs taken from 24 May showed a low water level in the lake of the Main Crater and that the lake was milky gray in color. The surface around the crater was covered in recently erupted white ash deposits (figure 41). Intermittent eruptions were reported during 25-26 May at 1435, 2230, 0453, and 0704, the strongest of which occurred at 1435. Small, low-frequency volcanic earthquakes and VT-type events proximal to the crater continued, but in smaller numbers. Intermittent degassing has persisted and there was a notable decrease in the lake level. Several eruptions were detected at 1357, 2248, and 2348 on 26 May and at 0221 and 0632 on 27 May. An eruption at 2135 on 27 May ejected incandescent material above the crater and generated a significant lahar on the N flank of the volcano that descended the Pénjamo River. The resulting gas-and-steam plume exceeded the field of view of the webcam, so OVSICORI-UNA estimated that it reached at least 3.5 km above the crater, and possibly as high as 4 km above the crater. On 29 May an eruption was observed at 0244 that ejected incandescent material and generated a lahar on the N flank of the volcano that descended the Pénjamo River.

Figure 41. Aerial photo of the Main Crater of Rincón de la Vieja taken on 24 May 2023. The active crater is covered in white ash deposits from recent eruptions. Courtesy of OVSICORI-UNA.

During 1-2 June five moderate phreatic eruptions were detected, the most energetic of which occurred at 0902 on 2 June. An accompanying gas-and-steam plume rose 1.5 km above the crater (figure 42). Seismographs intermittently recorded volcanic tremors. According to the Sentinel satellite, a sulfur dioxide plume weighing 37 tons was recorded on 2 June that drifted E. On 3 June at 0624 a gas-and-steam plume rose 1.5 km above the crater and drifted W. Some low magnitude VT-type events were recorded S of the Pailas area of the Rincón de la Vieja Volcano National Park. Sulfur dioxide emissions were recorded at average values of less than 300 tons/day. On 4 June at 0526 an eruptive plume reached 500 m above the crater, accompanied by a vertical gas-and-steam plume that rose 3.5 km above the crater and drifted N. OVSICORI-UNA reported that ash remained in the crater and there were no reports of it falling outside the summit. An eruption was recorded at 1804 on 5 June generated a gas-and-steam plume that rose 1 km above the crater, based on data from webcams, seismographs, infrasound sensors, and ground observations made to the N of the volcano. On 6 June at 0259 a small phreatic eruption was recorded that produced a gas-and-steam plume 3 km above the crater and drifted NW. On 7 June at 0142 a small eruption lasted 14 minutes based on webcam images. The eruption consisted of two pulses, the second of which was slightly more energetic. One eruption was recorded at 1505 on 7 June, but due to weather conditions the height of the plume could not be observed. Another eruption at 0942 on 8 June generated a gas-and-steam plume that rose 2.5 km above the crater. A moderate phreatic eruption was recorded at 1810 on 8 June that ejected lake water with sediment to less than 100 m above the crater rim; the accompanying white gas-and-steam plume rose 3 km above the crater and drifted W. Small phreatic eruptions were recorded at 0357, 0521 and 0546 on 11 June, the latter two of which produced a white gas-and-steam plume that rose 1.5 km and 2.5 km above the crater, respectively and drifted W. On 18 June small eruptions were recorded at 0440 and 0904 and continuous degassing was observed. A small phreatic eruption at 1804 on 18 June generated a gas-and-steam plume that rose 2 km above the crater. Several small, low-energy eruptions were recorded during the night of 25-26 June, the last one of which was recorded at 0547 that generated a gas-and-steam plume that rose 1.5 km above the crater.

Figure 42. Image showing a white gas-and-steam plume rising 1.5 km above the crater of Rincón de la Vieja at 1907 on 2 June 2023. Courtesy of Carol Quesada via OVSICORI-UNA.

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/, https://www.facebook.com/OVSICORI/).