Small gas-and-ash explosions during March and May 2023

San Miguel in El Salvador is a broad, deep crater complex that has been frequently modified by eruptions recorded since the early 16th century and consists of the summit known locally as Chaparrastique. Flank eruptions have produced lava flows that extended to the N, NE, and SE during the 17-19th centuries. The most recent activity has consisted of minor ash eruptions from the summit crater. The current eruption period began in November 2022 and has been characterized by frequent phreatic explosions, gas-and-ash emissions, and sulfur dioxide plumes (BGVN 47:12). This report describes small gas-and-ash explosions during December 2022 through May 2023 based on special reports from the Ministero de Medio Ambiente y Recursos Naturales (MARN).

Activity has been relatively low since the last recorded explosions on 29 November 2022. Seismicity recorded by the San Miguel Volcano Station (VSM) located on the N flank at 1.7 km elevation had decreased by 7 December. Sulfur dioxide gas measurements taken with DOAS (Differential Optical Absorption Spectroscopy) mobile equipment were below typical previously recorded values: 300 tons per day (t/d). During December, small explosions were recorded by the seismic network and manifested as gas-and-steam emissions.

Gas-and-ash explosions in the crater occurred during January 2023, which were recorded by the seismic network. Sulfur dioxide values remained low, between 300-400 t/d through 10 March. At 0817 on 14 January a gas-and-ash emission was visible in webcam images, rising just above the crater rim. Some mornings during February, small gas-and-steam plumes were visible in the crater. On 7 March at 2252 MARN noted an increase in degassing from the central crater; gas emissions were constantly observed through the early morning hours on 8 March. During the early morning of 8 March through the afternoon on 9 March, 12 emissions were registered, some accompanied by ash. The last gas-and-ash emission was recorded at 1210 on 9 March; very fine ashfall was reported in El Tránsito (10 km S), La Morita (6 km W), and La Piedrita (3 km W). The smell of sulfur was reported in Piedra Azul (5 km SW). On 16 March MARN reported that gas-and-steam emissions decreased.

Low degassing and very low seismicity were reported during April; no explosions have been detected between 9 March and 27 May. The sulfur dioxide emissions remained between 350-400 t/d; during 13-20 April sulfur dioxide values fluctuated between 30-300 t/d. Activity remained low through most of May; on 23 May seismicity increased. An explosion was detected at 1647 on 27 May generated a gas-and-ash plume that rose 700 m high (figure 32); a decrease in seismicity and gas emissions followed. The DOAS station installed on the W flank recorded sulfur dioxide values that reached 400 t/d on 27 May; subsequent measurements showed a decrease to 268 t/d on 28 May and 100 t/d on 29 May.

Figure 32. Webcam image of a gas-and-ash plume rising 700 m above San Miguel at 1652 on 27 May 2023. Courtesy of MARN.

Information Contacts: Ministero de Medio Ambiente y Recursos Naturales (MARN), Km. 5½ Carretera a Nueva San Salvador, Avenida las Mercedes, San Salvador, El Salvador (URL: http://www.snet.gob.sv/ver/vulcanologia).

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

April 1970 (CSLP 32-70)

Eruption during 30 March-4 April causes ashfall 10 km away

Card 0902 (03 April 1970) Renewed activity on 30 March

Reports arriving from the city of San Miguel state that Chaparraspique [Chaparrastique] volcano has again become active, at times intensively. It was reported that the first signs that the volcano had become active were noticed at 1300 GMT 30 March and that it continues to be covered by a layer of smoke and mist, which indicates that it may become active again at any moment, forcing the authorities to evacuate the area populated by hundreds of families.

Card 0904 (07 April 1970) Ashfall extends 10 km from the summit; crater visit

"Ash erupted from the crater of San Miguel volcano, El Salvador, on 30 March 1970, at 0230, 0700, and 0800. The first ash eruption, accompanied by a loud noise heard four kilometers away, reached a height of 400 m above the crater floor, according to Sr. Santiago Ventura Arevalo, Secretary of the Finca la Placita, San Miguel, and a report in La Prensa (San Salvador) of 31 March 1970. Ash fell in a fan-shaped area extending at least 10 km NE of the summit. From measurements of ash deposits three and four days after the eruption, the minimum volume of ash is estimated to be 75,000 m³. At the time of the ascent into the crater on 4 April 1970, there was ash in the crater floor to a depth of 15 mm. Infrequent ash eruptions up to 100 m high were observed on 3 and 4 April from the deep chimney in the summit crater. A land-based infrared radiation thermometry survey on 4 April detected no anomalous heat patterns on the NW side of the cone."

Information Contacts:
Card 0902 (03 April 1970) San Salvador Domestic Service, El Salvador.
Card 0904 (07 April 1970) Richard E. Stoiber, Dartmouth College, USA; Ian Lange, Fresno State College; Richard Birnie, USA Cold Reg. Research and Engineering Laboratory.

December 1976 (NSEB 01:15) Cite this Report

Explosive eruption began on 2 December; ashfall damages crops

An explosive eruption of San Miguel was reported to have begun during the morning of 2 December. "Smoke", loud noises, and a sulfur smell were reported from the nearby town of San Miguel. By 9 December, the eruption cloud was visible from a considerable distance and ashfalls had caused some crop damage. A few persons were evacuated. Although "fire" was reported by the press, only a white cloud was seen by Richard Stoiber during a 9 December overflight. The last activity at San Miguel was the eruption of more than 75,000 m³ of ash beginning 30 March 1970.

Further Reference. Martinez, M.A., 1977, The eruption of 2 December, 1976 of San Miguel Volcano, Republic of El Salvador, Central America: Centro de Investigaciones Geotécnicas, San Salvador, 4 p.

Information Contacts: AFP; Sercano Radio Network; R. Stoiber, Dartmouth College.

March 1977 (BVE 17)

New spatter cone in the central crater

The following is based on a report in BVE 17. After 6 years of dormancy since the last activity in 1970, San Miguel started erupting at about 0030 on 2 December 1976. The eruption was characterized by lava fountains. On 12 December the eruption stopped, and fumarole activity was seen around a new spatter cone in the bottom of the central crater. Fifty-six days later, the activity started again during the night of 28 February and lasting until 1 March 1977.

Information Contacts: M.A. Martinez, Centro de Investigaciones Geotécnicas, San Salvador, El Salvador.

December 1980 (SEAN 05:12) Cite this Report

Small vapor plume

During a flight over El Salvador by Dartmouth geologists, a small, continuous vapor plume rose from the summit crater.

Information Contacts: R. Stoiber, S. Williams, R. Naslund, L. Malinconico, and M. Conrad, Dartmouth College.

January 1986 (SEAN 11:01) Cite this Report

Ash emission and seismicity

Weak steam and ash emission accompanied by frequent small earthquakes began in November and were continuing in early February. The eruption deposited a thin layer of ash near the summit. Geologists installed a telemetering seismometer about 1 km from the summit, recording about 200 discrete microseismic events/day from November through much of January. By the end of January, seismic activity had declined somewhat, to 100-120 events/day.

Information Contacts: José González and Salvador Jesus, Dept de Sismología, Centro de Investigaciones Geotécnicas, San Salvador, El Salvador; David Harlow, USGS.

March 1995 (BGVN 20:03) Cite this Report

Increased seismicity and minor ashfall near the crater

New fumaroles were found near the central vent in early January, followed by an increase in seismic activity from an average of 20-30 events/day. On 8 February there were 52 recorded earthquakes. Seismicity increased to 73 events on 19 February, 100 on the 20th, and peaked at 267 on the 21st. This activity then declined on 22 February to an average of 76 events/day, a rate which continued through at least 24 March. Minor ashfall was reported on 23 March within ~100 m of the crater.

The Centro de Investigaciones Geotécnicas (CIG) concluded that this activity was no cause for alarm, but they would increase their monitoring efforts. The population at risk from an eruption with significant ashfall is a mix of urban and rural residents. The city of San Miguel (10 km NE) has a population of ~150,000, and the rural zone that would likely be affected has a population of ~100,000.

Information Contacts: Jorge Alberto Rodríguez Deras, Director, Centro de Investigaciones Geotécnicas, San Salvador, El Salvador.

February 2002 (BGVN 27:02) Cite this Report

Minor gas-and-ash emission in January 2002; summary of earlier activity

On 16 January 2002 a gas-and-steam plume containing a little ash rose with a mushroom-like profile a few hundred meters above the summit crater of San Miguel. During a visit to the summit on 28-29 January, Demetrio Escobar and a group from Michigan Technological University observed a thin layer of ash inside the summit crater produced by the 16 January event. COSPEC measurements of 100 metric tons/day were recorded at a time that the plume from San Miguel rose about 100 m above the crater rim. Long-period earthquakes, volcanic tremor, and explosion events were recorded at San Miguel in late January and February. This style of increased seismicity and gas emission is within the range of normal activity at San Miguel, and the Servicio Geologico de El Salvador concluded that this activity was no cause for alarm, but planned the installation of more telemetry seismic stations.

The events at San Miguel received extensive coverage in the local press, which had previously reported ash eruptions on 25 December 2001. However, during field studies around the volcano during 4-6 January by Escobar and Craig Chesner, extensive interviews of local residents revealed that the report of ashfall was incorrect. Vigorous steam plumes from the volcano had been confused with fly ash from the burning of sugar cane fields.

Summary of previous activity. Intermittent periods of vigorous steam-and-gas emission from San Miguel have been commonly reported in recent years. On at least two occasions, in early 1995 and at the end of December 1997, minor gas-and-ash emissions had occurred. Although discussed previously in BGVN 20:03, Escobar provided additional information on the 1995 event. Elevated seismicity and gas emission occurred at San Miguel from 26 December 1994 to 11 January 1995.

On 12 January 1995 Escobar visited the volcano and heard jet-like sounds at the crater, along with probable small explosions. Gas emission formed a plume about 100 m high that was blown to the SW. Escobar found 3-4 mm of ash in the summit crater. He also observed fine ashfall deposits on the SW flank and measured up to 2-3 mm of gray ash on the NW flank at a distance of 3 km from the crater. Residents reported that the ashfall caused damage to coffee plantations.

Elevated seismicity continued, and similar conditions were observed during a visit to the crater in February 1995. On 20 March the telemetered seismic station worked only 11 hours, but registered 48 events, including 4 volcanic tremor events and 39 LP events. Three events were felt by residents living near the volcano.

At the time of a 23-24 April 1995 visit, residents reported they had not felt any seismic events, but that ashfall had occurred on 19 April following a rainstorm. Seismicity returned to normal in May of that year.

On 17 June 1996 seismic stations registered a seismic swarm at San Miguel volcano during 1128 to 1333, consisting of 24 volcano-tectonic events with a maximum magnitude of 2.5. No felt events were reported.

Residents living near the volcano reported minor ashfall on 31 December 1997, although no geologists were present to document the ashfall distribution. On 13 January 2000, volcanic tremor was recorded for 15 minutes. On 25 January, 17 minutes of tremor occurred, along with explosion events. During May tremor was recorded for a total of 2.5 hours, and intervals of long-period earthquakes, volcanic tremor and explosion events occurred later in the year.

On 26 August 2000 a lahar from San Miguel damaged houses and a highway N of the volcano, and a similar event on 6 September 2001 affected a highway. Previously lahars had destroyed or damaged houses and highways N of the volcano on 7 May 1985 and 28 September 1992.

Information Contacts: Carlos Pullinger and Demetrio Escobar, Seccion Vulcanologia, Servicio Geológico de El Salvador, c/o Servicio Nacional de Estudios Territoriales, Alameda Roosevelt y 55 Avenida Norte, Edificio Torre El Salvador, Quinta Planta, San Salvador, El Salvador; Gustavo Chigna, Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hidrologia (INSIVUMEH), Ministero de Communicaciones, Transporto, Obras Públicas y Vivienda, 7a. Av. 14-57, zona 13, Guatemala City 01013, Guatemala (URL: http://www.insivumeh.gob.gt/); Bill Rose, Liset Rodríguez, Gustavo Chigna, Otoniel Matías, Janelle Byman, Elly Bunzendahll, Ivonne Branan, and Matt Watson, Michigan Technological University, Houghton, MI 49931 USA.

October 2006 (BGVN 31:10) Cite this Report

Restlessness persists during 2005-6; heavy tropical rains trigger lahars

According to El Salvador's Servicio Nacional de Estudios Territoriales (SNET) activity levels at San Miguel have generally remained similar to those during January 2002 when a minor plume rose above the summit crater (BGVN 27:02). The volcano's vigor continued into at least October 2006 at a level slightly at or above the base line of normal activity.

Recent publications have discussed the volcano and its lahar-hazard potential (Escobar, 2003; Chesner and others, 2003; Major and others, 2001). Figures 1 and 2 are taken from the latter publication.

Figure 1. Index map indicating El Salvador's volcanic front and the location of volcan San Miguel. Major cities are also shown (circles). From Major and others (2001).

Figure 2. The lahar hazard map of San Miguel depicts likely lahar paths, which are shown as colored or shaded areas. The contour interval is 20 m; the urban center ~ 11 km NE of the summit is San Miguel. From Major and others (2001); their plate 1, cropped, highly reduced, and excluding the key.

In January 2005 observers saw new fumaroles as well as small landslides on the N and SW wall of the crater. The accumulation of mass-wasted material in the crater led to a rise in the elevation of the crater floor.

During February 2005, weak fumaroles and small rock landslides persisted in the central crater. Digital sensors installed there recorded fumarolic temperatures in real time. On the outer portions of the cone the terrain is steeply sloping and contains prominent gullies (figure 3).

Figure 3. A photo of San Miguel taken from the N on 22 February 2005 showing the steep sides of the upper slopes and the incised drainages there. Although much of the area on the volcano is rural, hazards could easily affect 40,000 residents living nearby. Courtesy of Servicio Nacional de Estudios Territoriales (SNET).

The SNET reports for March and April 2005 noted that the crater was structurally weak due to the fumarolic activity, ongoing rock alteration, occasional landslides, and fractures on the western plateau. Microseismicity had increased; but it did not exceed typical base-line levels. Workers at the Santa Isabel farm (finca) noted N-flank lahars after heavy rains during March. The N flank contains abundant fine-grained volcanic deposits of the sort easily swept away during times of heavy rain.

Intense rains during May 2005 were associated with tropical storm Adrian (over an unstated interval the meteorological station near the volcano, San Miguel UES, recorded 428 mm of rainfall). As a result of the deluge, fumarolic activity from the crater increased. The crater walls remained intact, but eroded material previously deposited in the central crater that was poorly consolidated had to some degree stabilized. Substantial further compaction, settling, or collapse in the central crater seemed to have ceased by July 2005. During August 2005 the crisis at volcan Santa Ana forestalled visits to San Miguel.

A spike in seismic activity occurred during August 2005, with 7,048 long-period earthquakes, compared to July 2005, with 2,239 long-period earthquakes. SNET reports noted that based on monitoring, San Miguel generally remained within its base-line of normal behavior during the reporting interval. Figure 4 shows a histogram of long-period and volcano-tectonic events from the SNET reports for the interval September 2005-June 2006.

Figure 4. A plot of seismicity at San Miguel during September 2005-June 2006. Courtesy of SNET.

On 14 September 2005 a visiting group (OIKOS- Soliradaridad Internacional) made a trek to the summit and videotaped the scene there. SNET said the video disclosed a lack of significant changes in the crater; however, they saw debris-flow deposits in summit drainages on the volcano's outboard flanks. The visitors described both sounds of degassing and moderately intense odors of H₂S. During the course of September the seismic system recorded several minutes of tremor.

The October 2005 SNET report noted that workers at the plantation Santa Isabel noted N-slope lahars associated with rainfall. The lahars were also described as small debris flows; they descended from the high-elevation headwater areas, which are steep sided and narrow. The November report commented about the quantity of debris-flow material accumulating at the base of some N-flank channels. The same report also mentioned that moderate degassing was seen in the crater leaving areas of abundant sulfur, which appeared as yellow zones in one or more fumarolic areas.

The November 2005 report of SNET also discussed substantial landslides inside the crater that were followed by widening of the funnel-shaped area of collapse in the central crater. The landslides had left three distinct perched remnants of the crater floor (small terraces) at various elevations on the crater walls. The crater's western plain (one such terrace of the sort mentioned above) was stable but showed areas of subsidence (figure 5).

Figure 5. (top) A photo of San Miguel taken on 16 November 2005 showing the 'western plain' of San Miguel's crater (a terrace representing a remnant of a former crater floor). A considerable portion of the remaining terrace is in the process of subsidence (slumping). (bottom) A photo of the same area taken on 15 February 2006 (looking S). A zone of local subsidence, a pit along the head scarp, appears in the foreground but the subsidence also includes the region to the left of the large arcuate area extending well beyond the pit and still conspicuous in the upper left edge of the photograph. Courtesy of SNET.

Lahar monitoring during December 2005 disclosed erosion of easily mobilized cinders and scoria material on the N to NW flanks during the previous wet season. December seismicity was elevated, but cracks in the crater changed little compared to previous measurements. A field team visited the summit on 11 January 2006 and again in February and found few substantive changes in the crater. On the ascent route during January, the team saw a small recent "fall of material" reaching 40 cm thick. Some fumaroles discharged yellowish gases. During February the team conducted measurements of cracks on the western plain but found few changes, suggesting the headscarp had moved little if at all. February and March tremor episodes were centered at ~ 5 Hz and lasted 1-3 minutes.

The March 2006 SNET report noted small rockslides on the crater's N and S sides and, with the beginning of the rainy season in March 2006, there was a potential for the development of lahars. During the March visit the team found abundant granular material in the gullies on the NW flank, judged to be the result of debris flows. Monitored cracks remained stable.

With the arrival of the wet season in April, lahars and enhanced fumarolic output became apparent. One debris flow intersected a highway. On 23-24 April, 105 mm of rain was recorded at plantation (finca) Santa Isabel. Figure 6 shows the results of one lahar which left a trail of debris during the rainy interval. Earlier in the month on the 16th, a tremor or multi-phase episode lasted over an hour.

Figure 6. A San Miguel photo showing a part of the freshly scoured upslope channel in the Gato erosional gully. The material deposited in the channel consisted of reworked volcanic rocks and must have descended as a small lahar or debris flow. Several such flows occurred during heavy late-April rains at the start of the rainy season, a few days before this picture was taken. Courtesy of SNET (from their April 2006 report).

In April 2006, an increase in fumarole degassing within the crater and small landslides contributed to the instability of the deposits on the NW flanks of the volcano. Steam emanated from the fumaroles occasionally forming a weak column that reached the edge of the crater. There was a slight increase in seismicity throughout the month. Seismic activity increased in March and April 2006 (figure 4). Rocks in the crater show intense hydrothermal alteration with a yellowish reddish color. Small rock landslides were observed in the N and S zone of the crater.

During June 2006, the temperature of the fumaroles, opening of cracks and the gas discharge by the crater of the volcano, remained stable. There was an increase of small landslides within the crater. The analysis of the seismicity indicates that the volcano is slightly above its base line of normal behavior. New landslides and cracked rock were observed in the walls of the crater (figure 7). Rains have transferred volcanic material down the NW flank. Seismicity gradually increased in both frequency and magnitude beginning on 16 June. 47 VT earthquakes and 7,505 LP earthquakes were recorded, an amount that surpasses those registered in May; but smaller than those registered in March and April (figure 4).

Figure 7. San Miguel's S crater wall exposes zones of altered and fractured rocks. A planar zone of structural weakness appears towards the right. Photo taken on 22 June 2006. Courtesy of SNET.

During July 2006, stability continued with respect to fumarole temperatures, crack openings, and gas emissions around the crater. However, the seismicity increased by ~ 70%. Small and sporadic landslides took place inside the crater off the SE to SW walls. Intense hydrothermal alteration in the NW wall was also observed. SNET did not report any lahars during July 2006; however intense rains have continued to remove volcanic material from the NW flanks. The fumarolic field gave off weak emissions.

In August 2006, the monitored parameters such as fumarole temperature, crack opening, and visual estimates of gas discharge maintained normal levels. The seismicity diminished significantly in relation to July.

During September 2006, San Miguel reached a low level of activity. There were no significant changes in the morphology of the volcano as reported in previous months. At the S wall, there were evidence of small rock slides.

A sudden increase in seismicity occurred on 9 October 2006. Contact was made with other observatories and it was determined there were no landslides or rock falls associated with the event. Seismic increases such as 9 October had previously occurred, particularly on 19 June 2003 and from 2-6 May 2004. The 9 October increases were attributed to gas emission from the crater.

References. Chesner, C. A., Pullinger, C., Escobar, C. D., 2003, Physical and chemical evolution of San Miguel Volcano, El Salvador. GSA Special Paper 375.

Escobar, C.D., 2003, San Miguel Volcano and its Volcanic Hazards; MS thesis, Michigan Technological University, December 2003. 163 p.

Major, J.J.; Schilling, S.P., Pullinger, C.R., Escobar, C.D., Chesner, C.A, and Howell, M.M., 2001, Lahar-Hazard Zonation for San Miguel Volcano, El Salvador: U.S. Geological Survey Open-File Report 01-395. (Available on-line.)

Information Contacts: Carlos Pullinger, Seccion Vulcanologia, Servicio Geológico de El Salvador, c/o Servicio Nacional de Estudios Territoriales, Alameda Roosevelt y 55 Avenida Norte, Edificio Torre El Salvador, Quinta Planta, San Salvador, El Salvador (URL: http://www.snet.gob.sv/Geologia/Vulcanologia/).

September 2007 (BGVN 32:09) Cite this Report

Background seismicity since October 2006; crater visit in July 2007

A sudden increase in seismicity occurred on 9 October 2006 but no landslides or rock falls were associated with the event and it was attributed to gas emissions in the crater (BGVN 31:10). This report carries on from 9 October 2006.

During the morning of 10 October 2006, seismic activity declined to a continuous vibration with an amplitude that oscillated between 50 and 75 RSAM (real-time seismic amplitude measurement) units. This condition continued until 0600 on 11 October, when the seismicity increased to 125 continuous RSAM units.

The responsible authorities issued an alert that encompassed an area within 4 km from the center of the crater. Because of the elevated energy level of seismicity relative to the previous activity, the National Service of Territorial Studies elected to monitor the volcano and report developments to the National System of Civil Defense.

As of 15 October 2006, the level of activity at San Miguel was considered to be moderate, implying the possibility of an eruption sometime in the next several months. The civil defense authorities established a Yellow alert level (phase 3) for the area within 4 km of the crater center but later reduced it to Green. Around 15 October the RSAM continued to vary from 8 units to 45 units. During the preceding 24 hours, 55 earthquakes were registered; however, none were noticed by the local population. Sulfur dioxide (SO₂) fluxes reached 150-250 metric tons per day, which was considered a low level. On 16 October, tremor fluctuated between 45 and 50 units, and 25 earthquakes were recorded but not felt by residents.

The period from the October 2006 activity through July 2007 was essentially devoid of any abnormal variations in seismicity, volcanism, or elevated gas emissions.

On 4 July 2007, volcanologists from Servicio Nacional de Estudios Territoriales (SNET) and Michigan Technological University climbed San Miguel to make observations and take fumarole temperatures. The volcano remained at a low level of activity. The crater morphology and the intensity and location of fumaroles within the crater remained similar to that observed in recent visits (e.g., October 2006 BGVN 31:10). The main fumarolic area was near the bottom of the crater on the S wall (figure 8). Other sparse fumaroles were present, with most clustered near the crater bottom and on the crater's W wall.

Figure 8. View of the crater at San Miguel, looking S on 4 July 2007. The whitish area in the bottom right of the photo reflects steaming from the main fumarole field. Courtesy of Servicio Nacional de Estudios Territoriales (SNET) and Michigan Technological University.

Fumarole measurements: Temperatures were measured at two fumarolic areas on the upper W crater wall (figure 9). These are visited by SNET on a regular basis and comprise the only fumaroles safely accessible from the rim. Temperatures at fumaroles 1 and 2 were 67°C and 57°C, respectively. The gas lacked any sulfurous smell, suggesting water vapor only. These fumarole temperatures are similar to those measured in recent visits.

Figure 9. View of the W side of San Miguel's crater, taken from the N rim. Fumaroles 1 (F1) and 2 (F2) are in the right central portion of the image. Courtesy of Servicio Nacional de Estudios Territoriales (SNET) and Michigan Tech University.

References. Chesner, C.A., Pullinger, C., Escobar, C.D., 2003, Physical and chemical evolution of San Miguel Volcano, El Salvador. GSA Special Paper 375.

Escobar, C.D., 2003, San Miguel Volcano and its Volcanic Hazards: MS thesis, Michigan Technological University, December 2003, 163 p.

Major, J.J., Schilling, S.P., Pullinger, C.R., Escobar, C.D., Chesner, C.A, and Howell, M.M., 2001, Lahar-Hazard Zonation for San Miguel Volcano, El Salvador: U.S. Geological Survey Open-File Report 01-395 (Available on-line).

Information Contacts: Eduardo Gutierrez, Demetrio Escobar, and Francisco Montalvo, Servicio Nacional de Estudios Territoriales (SNET), Km. 5 ½ carretera a Santa Tecla y Calle las Mercedes, contiguo a Parque de Pelota, Edificio SNET, Apartado Postal #27, Centro de Gobierno, El Salvador (URL: http://www.snet.gob.sv/); Matthew Patrick and Anna Colvin, Dept. of Geological and Mining Engineering and Sciences, Michigan Tech University, 1400 Townsend Drive, Houghton, MI 49931, USA.

August 2015 (BGVN 40:08) Cite this Report

Eruption on 29 December 2013 with 9.7-km-high ash plume

San Miguel volcano, known locally as Chaparrastique, is one of the most active in El Salvador. This report covers activity from August 2007 through December 2014. Information is taken from Servicio Nacional de Estudios Territoriales (SNET) unless otherwise noted. The last reported eruption of San Miguel occurred as a gas-and-steam plume with a small amount of ash on 16 January 2002 (BGVN 27:02). Between 2002 and July 2007 there were no eruptions, and seismicity remained within background levels, except for a single episode during 9-15 October 2006 (BGVN 32:09). Typical activity at the volcano during this time was characterized by small fumaroles and landslides within the central crater, and a few small lahars associated with heavy rainfall.

Between August 2007 and December 2013, the volcano had intermittent pulses of seismic and fumarolic activity, but no ash eruptions. With almost no advance warning, San Miguel erupted on 29 December 2013, and caused the evacuation of a few surrounding villages. It quickly returned to lower levels of activity in early 2014, but continued emitting small ash-bearing plumes intermittently through July of 2014.

Activity during August 2007-November 2008. At San Miguel, SNET monitors seismic activity, fumarolic activity, gas emissions, and nearby water chemistry. From August 2007 through November 2008, Real-time Seismic Amplitude Measurements (RSAM) values (typically between 12 and 50 units) and other measurements rarely exceeded baseline levels. Rapid fluctuations of internal gas pressure, measured by RSAM, suggested continued degassing of the volcano. The largest variation from background levels occurred in the second week of October 2007 when RSAM values reached levels between 100 and 400 units for 5 days before dropping to background levels. Moderate to loud cracks and pops from gas releases were common sounds in the summit area. No reports were found for the December 2008-May 2009 period.

Activity during June 2009-November 2013. Typical seismic behavior during this time, according to SNET monthly reports, included sporadic elevations in RSAM values lasting for a few days every few months; values were usually in the 100-300 unit range, with periodic spikes to 660 units. Small earthquake swarms also occurred a number of times (June and July 2009, July and August 2010, March and April 2011, July and August 2011, November 2011, March and April 2013, and September 2013). Most of the swarms originated below the NNE flank.

Temperature measurements inside the crater varied during this period as well. For example, in December 2010 the temperature, as measured in the fumarole field with a thermal camera, fluctuated between 82 and 112°C, and in March of 2011, infrared images revealed variations between 112 and 121°C. The temperature in the main fumarole field was 134°C in November of 2011, which was 13°C higher than historical averages. By May of 2013, however, fumarole temperatures were back down to 70-87°C.

SO₂ emission rate variation during this period was very low. A mild sulfur odor was occasionally reported by SNET geologists on visits to the summit. Normal SO₂ values for San Miguel range between 100 and 437 tons/day, and the recorded values only exceeded this range consistently in May, July, and August 2013 (with values up to 900 t/d).

Fumarolic activity increased and decreased periodically during this time reflecting variable degassing of the volcano. SNET reports the noise level of the fumaroles as an indication of activity; most of the time they sounded like a "pressure cooker," and occasionally the noise levels increased to those like "a jet engine." Intermittent steam plumes were observed rising to 100 or 200 m above the crater.

No significant lahars were reported during the periods of high precipitation, although a number of fresh debris deposits (up to 20,000 cubic meters in estimated size) were observed in drainages near and across the access road to the volcano during the rainy seasons, especially in June and July 2013.

Various indicator values at the volcano all remained at or near background values through the middle of December 2013.

Eruption of 29 December 2013. A special report by MARN (Ministerio de Medio Ambiente Y Recursos Naturales) noted the sudden eruption of San Miguel at 1030 local time on 29 December (figure 10). The main eruptive phase lasted until 1300. Washington VAAC reported that the ash plume rose to an altitude of 9.7 km (32,000 ft) and drifted ENE at altitudes above 6 km and W at lower altitudes (figures 11, 12).

Figure 10. San Miguel volcano (Chaparrastique) erupts, 29 December 2013. Courtesy of REUTERS/Stringer.

Figure 11. Volcanic ash from the eruption traveled more than 70 km downwind to the SW on 29 December 2013. It also traveled ENE of the volcano at higher elevations. Courtesy of NASA Earth Observatory (Jeff Schmaltz, LANCE/EOSDIS MODIS Rapid Response Team).

Figure 12. NOAA ash column map showing ash cloud dispersion in different directions. From the surface to 6 km, the cloud drifted W, and between 6 and 10 km the cloud drifted ENE.Courtesy of Washington Volcanic Ash Advisory Center.

On 31 December MARN geoscientists estimated that the eruption deposits around the crater were about 50 cm thick. There was no incandescense at the crater, increase in emissions, or an increase in fumarole temperature prior to the eruption. The seismic stations recorded a sudden and dramatic increase in RSAM values at the time of the eruption. Only a very small increase in the average RSAM values was observed mid-December (figure 13).

Figure 13. RSAM values from two seismic stations on the flanks of San Miguel for December 2013. They show a slight increase in average RSAM values in mid-December and the sudden spike on the morning of the eruption, 29 December. Figure from MARN Special Report.

The sudden spike in RSAM values on 29 December was similar to seven prior spikes that were not associated with eruptions between October 2006 and December 2013 (figure 14).

Figure 14. Daily average RSAM values from 1 January 2006 through 1 January 2014. Seven similar spikes of RSAM values during this time were not associated with eruptions. Figure from MARN special report.

Hourly RSAM values recorded on 29 December show an increase above background levels at 0700, only three and a half hours before the eruption. By 1300 values had dropped back below previous levels. SNET reported that sulfur dioxide flux increased from 637 tons per day on 29 December, peaking at 2200 t/day on 31 December.

Ashfall deposits were observed in Santiago de Maria (1 mm), Placitas (5 mm) and San Jorge (2 mm) (figure 15).

Figure 15. Ashfall hazard map with data points and simulation of the eruption from the central crater, with a volume of .51 km3 and a 10 km high plume. Figure from MARN Special Report.

Activity during 2014. RSAM values quickly dropped back to normal levels within hours after the 29 December eruption. In January light-gray gas plumes continued to rise 100-250 m above the crater and drift generally S and SW. A spike to 262 RSAM units was attributed to a magma injection on 23 January. Small magnitude earthquakes (179) continued in the volcano area in February. Average RSAM values steadily increased from mid-January to late February from values below 200 to over 400 before dropping back below 200 at the beginning of March 2014. White and light-gray plumes rising to 300 m were reported in February and March. A small explosion occurred at 1644 on 12 February, but no additional information was given by MARN. Moderate SO₂ emissions were also reported during this time. Decreasing seismicity was recorded in March and April.

SNET reported that on 12 April moderate-to-strong gas plumes rose from the crater and drifted SW. The biggest plume occurred at 1607 and rose 400 m. Images recorded by a webcam showed that the plumes had dark tones, suggesting small amounts of ash. Another plume, rising between 400 and 600 m was recorded on 29 April with the thermal camera, but visual observations were obscured by weather conditions.

The volcano was more active from May to July 2014 with at least nine episodes of ashfall reported. The firstwas observed in areas between the Parque de las Placitas and Placitas on 10 May with deposits less than 1 mm thick. From 18-20 May, an increase in the frequency and magnitude of gas emissions, rumbling in the crater, and small explosions were followed by juvenile ashfall. Less than 1 mm of ash fell about 9 km to the WSW of the volcano in San Jorge on 18 May. On 19 and 20 May the webcam recorded periodic pulses of gray gas plumes that rose 300 m and drifted W. During 22-23 May MARN reported that heavy rains at San Miguel caused lahars that originated from the upper part of the volcano and were heard by residents. Seismicity and gas output both remained high during this time, with gas plumes rising 300 m and drifting W. Locals observed gas plumes with occasional minor amounts of ash rising from the crater, especially after rainfall. Rumbling was also reported. The RSAM average values in late May and June were higher than those recorded during the 29 December and 12 February eruptions, raising concerns of a new eruption.

Strombolian activity in June and July was characterized by sudden fluctuations in seismicity (figures 16). Ash emissions during 4, 5 and 6 June caused millimeter-scale ash deposits in areas 3 km N, NW and SW of the crater. On 8 June, 1 mm of ashfall was reported on the N flank. An inspection of the crater on 10 June revealed strong degassing with plumes rising to 200 m, 2 cm of gray ash deposits, and the temperature of the fumaroles at 200°C. Ash emissions were also reported 28, 29 and 30 June. In July seismicity at San Miguel remained intermittently high, especially beneath the N flank, and RSAM reached over 1000 units for 10 consecutive days. A small explosion on 13 July at 0941 sent ash to the WSW. The following day, 1-mm-thick, dark-gray ash deposits were observed on the LACAYO farm, 4 km WSW. On 23 July at 1831 another ash cloud from a small explosion rose 400 m and drifted SW. Minutes later, local observers reported ashfall in the Cantons of La Morita (6 km SW) and Piedra Azul, and also in San Rafael Oriente (11 km SW). At 1806 on 25 July and 2018 on 27 July two additional smaller ash-and-gas explosions occurred. A longer gas-and-ash explosion, lasting fifteen minutes occurred on 28 July at 0615. Both the 27 and 28 July explosions rose between 300 and 500 m and drifted SW. Local and Civil Protection of San Jorge observers reported millimeter depth ashfall from these two events in the cantons of La Morita, La Piedrita, and La Ceiba, located WSW of the crater. These were the last reported ash emissions in 2014.

Figure 16. Average hourly RSAM values from the VSM seismic station and ash emissions (Emision de ceniza) for July 2014. Courtesy of SNET.

Activity at San Miguel showed signs of decreasing in August 2014; fewer days of elevated SO₂ and above-baseline RSAM were recorded, no ash emissions were reported, and only white or whitish gray steam or steam-and-gas plumes (interpreted by MARN as water vapor and SO₂ emissions) rose 100-200 m above the crater. Heavy rains during 10-11 August generated two landslides along the creeks that flow down the SW and NE flanks of the volcano, causing some damage to the small communities in the region, including San Jorge located 9 km WSW of the summit. SO₂ emissions and seismicity spiked briefly from 31 August to 5 September (RSAM spiked to 1113 on 2 September, SO₂ reached 1135 t/d). Mudslides caused minor damage in early September.

Through December 2014 activity remained slightly elevated with RSAM values ranging between 50 and 387 units (figure 17), pulses of SO₂ emissions spiking to 1739 t/day, gas plumes rising no more than 400 m above the crater, and smaller numbers of earthquakes. There were no reports of explosions with ash emissions, and no significant change in the temperature inside the crater during this time.

Figure 17. Hourly average RSAM values from the VSM Seismic Station from 20 December 2013, just prior to the 29 December eruption, through the end of December 2014. The vertical scale goes to 1400 RSAM. Average values decreased significantly after August 2014.

Information Contacts: Servicio Nacional de Estudios Territoriales (SNET), Km. 5 1/2 carretera a Santa Tecla y Calle las Mercedes, contiguo a Parque de Pelota, Edificio SNET, Apartado Postal #27, Centro de Gobierno, El Salvador 2283-2246 (URL: http://www.snet.gob.sv/); Reuters (URL: http://www.reuters.com/, http://news.yahoo.com/el-salvador-volcano-erupts-evacuations-ordered-194649069.html); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/)

July 2017 (BGVN 42:07) Cite this Report

Six small ash emission events during January 2015-June 2017

Volcán de San Miguel (Chaparrastique), in southern El Salvador, was active with several flank lava flows during the 17th-19th centuries, but recent activity has consisted of occasional ash eruptions from the summit crater. The beginning of the most recent eruption on 29 December 2013 resulted in a large ash plume that rose to 9.7 km altitude, and dispersed ash to many communities within 30 km of the volcano (BGVN 40:08). Intermittent ash plumes lasted through 28 July 2014. This report covers activity from January 2015 through June 2017, and describes six small ash emission events during this time. Information about San Miguel comes from the the Ministero de Medio Ambiente y Recursos Naturales (MARN) of El Salvador, and the Washington Volcanic Ash Advisory Center (VAAC).

Six ash-bearing explosions occurred at San Miguel between January 2015 and June 2017. Otherwise, minor seismicity and pulses of gas-and-steam emissions were the primary type of activity. The explosion on 26 January 2015 sent a plume 300 m above the crater, drifting SW. An explosion on 11 April 2015 resulted in an ash plume rising about 800 m above the crater that also drifted SW. Trace amounts of ash were emitted on 13 August 2015. The largest explosion of the period took place on 12 January 2016, when an ash plume drifting W caused ashfall as far as 25 km away, and the plume was ultimately visible as far as 300 km from the volcano. Incandescence was observed at the base of the 900-m-high eruptive column that appeared on 18 June 2016. Minor ash emissions were reported on 7 January 2017 drifting 130 km SW from San Miguel. Minor seismic swarms and steam-and-gas plumes were reported during February-June 2017.

Activity during 2015. After very little activity other than slightly elevated RSAM values since July 2014, a small ash-bearing explosion occurred on 26 January 2015 (figure 18). The ash plume rose about 300 m above the crater and drifted SW, dissipating quickly. Trace amounts of ash fell in the Piedra Azul area about 6 km SW of the crater.

Figure 18. Gas-and-steam emissions at San Miguel on 26 January 2015, after an ash-bearing explosion that occurred earlier in the day. Courtesy of MARN (Informe Especial No. 8. Continúa constante emanación de gases del volcán Chaparrastique January 27, 2015 at 11:21 am).

Another emission lasting for 20 minutes on 22 February 2015 sent a column of gas 300 m above the crater that dispersed to the SSW; no ash was observed. Occasional pulses of gas were reported during March rising 200 m above the summit crater. An explosion on 11 April 2015 resulted in an ash plume rising about 800 m above the crater and drifting SW. Local observers reported a millimeter of ashfall in the areas of La Piedra, Morita, and San Jorge, less than 10 km to the SW.

Occasional small pulses of gas that rose to about 200 m above the crater were typical behavior during May-July (figure 19). On 13 August 2015, the webcam captured a gas plume emission that contained minor amounts of ash and rose 200-300 m above the crater. A millimeter of ashfall was reported in San Jorge, and near the communities of Moritas and Piedrita to the SW. No emissions were reported during September, and the small pulses of gas observed during October did not exceed 200 m above the summit crater. No anomalous activity was reported during November, and small discrete pulses of gas were the only activity reported for December 2015.

Figure 19. Diffuse degassing from the summit crater at San Miguel on 24 June 2015. Courtesy of MARN (Informe Mensual de Monitoreo Volcánico Junio, 2015).

Activity during 2016. San Miguel began the year with what MARN described as a VEI 1 eruption of ash and gas on 12 January 2016 (figure 20). Periodic pulses of ash and gas lasting 3-5 minutes rose to less than 1,000 m above the crater and drifted WSW. Ashfall was reported from La Piedra, Moritas, La Placita, San Jorge, (all less than 10 km SW), San Rafael Oriente (10 km SW), Alegría (25 km NW) and Berlin in Usulután (21 km SW).

Figure 20. Ash eruption at San Miguel in the early morning of 12 January 2016. Image from the MARN webcam on the N side of the volcano. Courtesy of MARN (Informe Mensual de Monitoreo Volcánico Enero, 2016).

NASA Earth Observatory captured images of two pulses of ash from the 12 January eruption that show the changing direction of the plume (figure 21). The first image, taken at 1635 (UTC) shows the ash plume headed directly W. The second image, taken three hours later at 1935 shows the active plume drifting SW, with the earlier plume segment farther to the W over the Pacific Ocean. The Washington VAAC reported the ash emission at 2.4 km altitude (300 m above the summit crater) drifting WSW at 1745 (UTC). At that time, the denser part of the plume extended 45 km from the volcano and the diffuse, wispy plume extended 130 km WSW. By midday on 13 January, the Washington VAAC reported ongoing emissions and that the plume extended 300 km SW. The plume was no longer visible in satellite images by the end of the day.

Figure 21. Moderate Resolution Imaging Spectroradiometer (MODIS) sensors on NASA's Aqua and Terra satellites acquired these natural-color images of ash streaming from San Miguel on 12 January 2016. Terra captured the upper image at 1035 local time (1635 UTC) which shows the ash plume drifting W; Aqua captured the lower image three hours later at 1335 local time (1935 UTC), and it shows the SW drift of the plume with the older remnant to the W over the Pacific Ocean. Courtesy of NASA Earth Observatory.

Seismicity declined during 12-14 January 2016. On 15 January, local observers reported a millimeter of ash deposited in Las Cruces on the N flank. Gas emissions during 17-18 January were weak, only rising 150 m. At 0900 on 18 January, the emission plume became dark and drifted SW. By the next crater inspection on 19 February, MARN scientists noted only minor degassing from the summit crater.

Although a period of volcanic tremor occurred on 8 March 2016, only short pulses of gas were observed that did not rise more than 150 m above the crater. Another spike in seismicity occurred on 3 April, but no gas or ash emissions were observed. Otherwise, only minor pulses of gas issued from the crater during February through May. A seismic swarm indicating rock fracturing at depth on 31 May could have resulted in trace amounts of ash deposited within the crater, but cloudy weather prevented observations. A few pulses of gas were observed from the webcam other times during May.

Seismic activity increased significantly during the second week in June. An explosion in the early morning of 18 June 2016 lasted about 60 seconds, and sent an ash emission to about 900 m above the crater (figure 22). Incandescence was observed within the eruptive column, and the debris fell about 100 m down the N flank. Ashfall of less than half a millimeter was reported in the El Volcan area about 7 km NE of the crater. The volcanologist who examined the ash determined that it was juvenile material from a magmatic explosion. A continuous column of steam-and-gas issued from the crater until 29 June (figure 23). During this time, local observers and officials from the Civil Protection of San Jorge reported sulfur odors and slight acid rain damage to the vegetation in the La Morita, La Piedrita, La Ceiba, and LACAYO farms, located about 4 km W of the crater.

Figure 22. The pre-dawn eruption of 18 June 2016 at San Miguel photographed from the MARN webcam. The ash emission rose to about 900 m above the summit crater. Courtesy of MARN (Informe Mensual de Monitoreo Volcánico Junio, 2016).

Figure 23. Gas plumes from San Miguel during the second half of June 2016 caused acid rain damage to vegetation W of the volcano. Upper image from the MARN webcam taken on 24 June 2016. The lower image was taken at 0800 on 27 June near Las Moritas, about 5 km WSW of the crater by Antonio Saravia. Courtesy of MARN (Informe Mensual de Monitoreo Volcánico Junio, 2016).

Seismic activity was slightly elevated during the first half of July 2016, but otherwise only small pulses of gas were observed from the crater. Low-level activity continued from the summit crater during August. On 29 August, however, a seismic signal indicative of a lahar was noted near the VSM (Santa Isabel) seismic station, but no damage was reported. Periodic pulses of gases were noted during September 2016. A 20-minute-long seismic signal on 5 September indicated another lahar passing near seismic station VSM, but no damage was reported. GPS measurements during September indicated deformation of a few millimeters on the N flank. No explosions were reported during October-December 2016, only small plumes of steam and gas were observed (figure 24).

Figure 24. Steam-and-gas plumes blowing SW from San Miguel during December 2016. Image taken by the webcam located at the El Pacayal (Chinameca) volcano about 8 km S. Courtesy of MARN (Informe Mensual de Monitoreo Volcánico Diciembre, 2016).

Activity during January-June 2017. On 7 January 2017, the Washington VAAC reported minor volcanic ash emissions from San Miguel at 2.6 km altitude extending SW about 130 km from the summit. Mild degassing continued during February and March. A brief seismic swarm occurred on 17 April 2017, but no explosions of gas or ash were observed in the webcam. A strong gas plume rose 1.2 km above the crater rim on 27 April. Seismicity decreased during May. Other than small gas plumes, the only activity reported during June 2017 was a slight increase in seismicity beginning on 12 June and lasting to the end of the month.

Information Contacts: Ministero de Medio Ambiente y Recursos Naturales (MARN), Km. 5½ Carretera a Nueva San Salvador, Avenida las Mercedes, San Salvador, El Salvador (URL: http://www.snet.gob.sv/ver/vulcanologia); 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: http://www.ospo.noaa.gov/Products/atmosphere/vaac/, archive at: http://www.ssd.noaa.gov/VAAC/archive.html).

June 2018 (BGVN 43:06) Cite this Report

Intermittent small ash emissions between 14 January and 30 May 2018

El Salvador's San Miguel, also known as Chaparrastique, had six small ash emission events during January 2015-June 2017 (BGVN 42:07). New activity consisting of intermittent ash emissions began on 14 January and continued until 30 May 2018, reported below based on information provided by El Salvador's Servicio Nacional de Estudios Territoriales (SNET) and special reports from the Ministero de Medio Ambiente y Recursos Naturales (MARN).

SNET and MARN reported that during 14-17 January 2018 there were four gas-and-ash emissions from San Miguel that rose no higher than 300 m above the crater rim, at least one of which dispersed SW. The reports noted that prior to each emission seismicity decreased and then suddenly increased. MARN reported that during 25-26 January seismic tremor levels fluctuated between 75 and 179 RSAM (Real-time Seismic Amplitude Measurement) units per hour on average, slightly above normal (50-150 units).

On 19 February, MARN reported the beginning of sustained gas emissions along with small ash emissions. The plume did not exceed 350 m above and was displaced by winds to the SW. This activity was similar to the activity on 14-15 January 2018.

SNET reported on 2 March that gas plumes rose as high as 400 m above the crater rim during the previous week. Ash appeared in "gas pulse" emissions on 24, 26, and 28 February, and 1 March. RSAM values fluctuated between 70 and 179 units during 1-2 March. At 2200 on 5 March seismic amplitude began to increase, with RSAM values rising to as high as 318 units by 0600 on 6 March. A webcam recorded minor gas emission during 5-6 March. MARN reported that RSAM values fluctuated between 68 and 248 units, with an average of 156 during 8-9 March. Continued volcanic tremor during 9-16 March was noted, along with persistent low-level degassing from the central crater. Volcanic tremor levels during 15-16 March fluctuated between 77 and 203 RSAM units per hour, with an average of 124.

By early April, MARN had noted a decrease in activity. On 3 April it reported that RSAM levels varied between 46 and 87 units, with an average of 55. Activity increased briefly during 7-13 April, and MARN reported that periodic microseisms combined with changes in seismic tremor and gas pulses had increased significantly, reaching maximum values of 400 RSAM units in an average hour (figure 25).

Figure 25. RSAM values at San Miguel during 7-13 April 2018. Courtesy of Ministero de Medio Ambiente y Recursos Naturales (MARN).

Discrete earthquakes were detected between 13 and 17 April, and discontinuous volcanic tremor during 17-18 April was associated with weak, sporadic degassing from the central crater. Seismicity reached maximum values of 216 RSAM units in an average hour.

MARN reported that during 20-27 April volcanic tremor fluctuated between 37 and 106 RSAM units per average hour. Seismicity was low during 28 April-4 May, with RSAM between 39 and 61 units per hour.

In May MARN reported that the volcanic activity had declined compared to April. As of 18 May there was no change in volcanic activity, despite the seismic swarm that started on the night of 5 May felt in the municipalities of Chirilagua-Intipucá, 30 km SE. Average SO₂ emission rates were variable during 1 January-6 May 2018 (figure 26).

Figure 26. Sulfur dioxide emissions at San Miguel between from 1 January-6 May 2018. Courtesy of Ministero de Medio Ambiente y Recursos Naturales (MARN).

SNET reported a significant increase in the number of low- and high-frequency earthquakes beneath the crater beginning on 22 May. RSAM values fluctuated between 142 and 176 units during 30 May-1 June. Webcam images on 30 May showed a small gray gas emission.

Information Contacts: Servicio Nacional de Estudios Territoriales (SNET), Ministero de Medio Ambiente y Recursos Naturales (MARN), Km. 5½ Carretera a Nueva San Salvador, Avenida las Mercedes, San Salvador, El Salvador (URL: http://www.snet.gob.sv/ver/vulcanologia, http://www.marn.gob.sv/category/avisos/vulcanologia/).

March 2020 (BGVN 45:03) Cite this Report

Small ash emissions during 22 February 2020

San Miguel, locally known as Chaparrastique, is a stratovolcano located in El Salvador. Recent activity has consisted of occasional small ash explosions and ash emissions. Infrequent gas-and-steam and ash emissions were observed during this reporting period of June 2018-March 2020. The primary source of information for this report comes from El Salvador's Servicio Nacional de Estudios Territoriales (SNET) and special reports from the Ministero de Medio Ambiente y Recursos Naturales (MARN) in addition to various satellite data.

Based on Sentinel-2 satellite imagery and analyses of infrared MODIS data, volcanism at San Miguel from June 2018 to mid-February was relatively low, consisting of occasional gas-and-steam emissions. During 2019, a weak thermal anomaly in the summit crater was registered in thermal satellite imagery (figure 27). This thermal anomaly persisted during a majority of the year but was not visible after September 2019; faint gas-and-steam emissions could sometimes be seen rising from the summit crater.

Figure 27. Sentinel-2 satellite imagery of a faint but consistent thermal anomaly at San Miguel during 2019. Images with "Atmospheric penetration" (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Volcanism was prominent beginning on 13-20 February 2020 when SO₂ emissions exceeded 620 tons/day (typical low SO₂ values are less than 400 tons/day). During 20-21 February the amplitude of microearthquakes increased and minor emissions of gas-and-steam and SO₂ were visible within the crater (figure 28). According to SNET and special reports from MARN, on 22 February at 1055 an ash cloud was visible rising 400 m above the crater rim (figure 29), resulting in minor ashfall in Piedra Azul (5 km SW). That same day RSAM values peaked at 550 units as recorded by the VSM station on the upper N flank, which is above normal values of about 150. Seismicity increased the day after the eruptive activity. Minor gas-and-steam emissions continued to rise 400 m above the crater rim during 23-24 February; the RSAM values fell to 33-97 units. Activity in March was relatively low; some seismicity, including small magnitude earthquakes, occurred during the month in addition to SO₂ emissions ranging from 517 to 808 tons/day.

Figure 28. Minor gas-and-steam emissions rising from the crater at San Miguel on 21 February 2020. Courtesy of Ministero de Medio Ambiente y Recursos Naturales (MARN).

Figure 29. Gas-and-steam and ash emissions rising from the crater at San Miguel on 22 February 2020. Courtesy of Ministero de Medio Ambiente y Recursos Naturales (MARN).

Information Contacts: Servicio Nacional de Estudios Territoriales (SNET), Ministero de Medio Ambiente y Recursos Naturales (MARN), Km. 5½ Carretera a Nueva San Salvador, Avenida las Mercedes, San Salvador, El Salvador (URL: http://www.snet.gob.sv/ver/vulcanologia); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

December 2022 (BGVN 47:12) Cite this Report

Frequent explosions, gas-and-ash emissions, and elevated sulfur dioxide during November 2022

San Miguel in El Salvador, also locally known as Chaparrastique, is a broad, deep crater complex that has been frequently modified by eruptions recorded since the early 16th century. Flank eruptions have produced many lava flows, including several during the 17th to 19th centuries that extended to the N, NE, and SE. Eruptive activity during February 2020 consisted of small ash emissions with weak thermal anomalies (BGVN 45:03). This report covers explosive activity during November 2022 based on special reports from the Ministero de Medio Ambiente y Recursos Naturales (MARN) in addition to various satellite data.

According to MARN, an eruption at the central crater began on 15 November that consisted of intermittent phreatic explosions generating gas-and-ash emissions that generally rose 500 m above the crater (figure 30). By 1100 on 20 November a total of 62 phreatic explosions had been recorded, averaging 10 explosions per day. Seismicity was characterized by volcano-tectonic (VT) events, long-period (LP) earthquakes, and tremor (TR).

Figure 30. Webcam image showing a gas-and-ash plume from San Miguel on 21 November 2022. Courtesy of MARN.

Eruption plumes at 1336 on 18 November and at 1206 on 19 November rose as high as 1.1 km above the crater rim. Some explosion events were accompanied by crater incandescence during 15-20 November; Sentinel-2 infrared satellite imagery showed a small thermal anomaly at the summit crater on 17 and 22 November (figure 31). Energetic explosions were reported at 1302 and 1642 on 26 November and at 0718 and 0802 on 27 November that ejected hot rocks onto the flanks and generated eruption plumes that rose as high as 1.1 km above the crater rim. A total of 194 explosions were recorded during 15-29 November, with a daily average of 12. On 7 December MARN reported that seismicity had decreased, and no explosions had been detected since 29 November.

Figure 31. Sentinel-2 infrared satellite images showed a small thermal anomaly at the summit crater of San Miguel on 17 and 22 November 2022 (left and right). Images with "Atmospheric penetration" (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Sulfur dioxide measurements that were taken during explosive events showed higher levels of sulfur dioxide emissions. On 19 November, during one of the largest explosions, SO₂ emissions were 1,200 t/d, though on 21 November emissions were again lower at 378 t/d. During the more energetic explosions over 26-27 November, sulfur dioxide emission rates were again as high as 1,200 t/d, but they were otherwise 100-170 tons per day (t/d) on the 26th and 283 t/d on the 27th, which was lower than the typical baseline of 300 t/d. After 30 November sulfur dioxide emissions were again below baseline levels.

Information Contacts: Ministero de Medio Ambiente y Recursos Naturales (MARN), Km. 5½ Carretera a Nueva San Salvador, Avenida las Mercedes, San Salvador, El Salvador (URL: http://www.snet.gob.sv/ver/vulcanologia); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

June 2023 (BGVN 48:06) Cite this Report

Small gas-and-ash explosions during March and May 2023

San Miguel in El Salvador is a broad, deep crater complex that has been frequently modified by eruptions recorded since the early 16th century and consists of the summit known locally as Chaparrastique. Flank eruptions have produced lava flows that extended to the N, NE, and SE during the 17-19th centuries. The most recent activity has consisted of minor ash eruptions from the summit crater. The current eruption period began in November 2022 and has been characterized by frequent phreatic explosions, gas-and-ash emissions, and sulfur dioxide plumes (BGVN 47:12). This report describes small gas-and-ash explosions during December 2022 through May 2023 based on special reports from the Ministero de Medio Ambiente y Recursos Naturales (MARN).

Activity has been relatively low since the last recorded explosions on 29 November 2022. Seismicity recorded by the San Miguel Volcano Station (VSM) located on the N flank at 1.7 km elevation had decreased by 7 December. Sulfur dioxide gas measurements taken with DOAS (Differential Optical Absorption Spectroscopy) mobile equipment were below typical previously recorded values: 300 tons per day (t/d). During December, small explosions were recorded by the seismic network and manifested as gas-and-steam emissions.

Gas-and-ash explosions in the crater occurred during January 2023, which were recorded by the seismic network. Sulfur dioxide values remained low, between 300-400 t/d through 10 March. At 0817 on 14 January a gas-and-ash emission was visible in webcam images, rising just above the crater rim. Some mornings during February, small gas-and-steam plumes were visible in the crater. On 7 March at 2252 MARN noted an increase in degassing from the central crater; gas emissions were constantly observed through the early morning hours on 8 March. During the early morning of 8 March through the afternoon on 9 March, 12 emissions were registered, some accompanied by ash. The last gas-and-ash emission was recorded at 1210 on 9 March; very fine ashfall was reported in El Tránsito (10 km S), La Morita (6 km W), and La Piedrita (3 km W). The smell of sulfur was reported in Piedra Azul (5 km SW). On 16 March MARN reported that gas-and-steam emissions decreased.

Low degassing and very low seismicity were reported during April; no explosions have been detected between 9 March and 27 May. The sulfur dioxide emissions remained between 350-400 t/d; during 13-20 April sulfur dioxide values fluctuated between 30-300 t/d. Activity remained low through most of May; on 23 May seismicity increased. An explosion was detected at 1647 on 27 May generated a gas-and-ash plume that rose 700 m high (figure 32); a decrease in seismicity and gas emissions followed. The DOAS station installed on the W flank recorded sulfur dioxide values that reached 400 t/d on 27 May; subsequent measurements showed a decrease to 268 t/d on 28 May and 100 t/d on 29 May.

Figure 32. Webcam image of a gas-and-ash plume rising 700 m above San Miguel at 1652 on 27 May 2023. Courtesy of MARN.

Information Contacts: Ministero de Medio Ambiente y Recursos Naturales (MARN), Km. 5½ Carretera a Nueva San Salvador, Avenida las Mercedes, San Salvador, El Salvador (URL: http://www.snet.gob.sv/ver/vulcanologia).