Ash emissions, ashfall, and occasional lahars during February-July 2023

Cotopaxi is a steep-sided cone capped by nested summit craters, located in Ecuador. The largest cone is about 550 x 800 m. Explosive eruptions have been accompanied by pyroclastic flows and lahars, the latter of which frequently affect adjacent valleys. The most recent eruption began in October 2022 and consisted of ash plumes and ashfall (BGVN 48:02). This report covers activity during February through July 2023, which was characterized by similar activity. Information comes from Ecuador's Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN) and the Washington Volcanic Ash Advisory Center (VAAC).

Activity during February consisted of constant gas-and-steam emissions that rose 200-2,500 m above the crater and drifted in multiple directions, and seismicity that was characterized by daily tremor events associated with emissions (TR) and long-period events (LP), and less frequent volcano-tectonic events (VT) and very long-period events (VLP). The Washington VAAC reported during 31 January to 4 February that gas-and-ash emissions rose 1.1-2.5 km above the summit and drifted in multiple directions. Beginning at 0100 on 2 February IG reported an increase in seismic signals associated with ash emissions that rose 1.3 km above the summit and drifted NW (figure 25). There were reports of ashfall in the N part of the Cotopaxi National Park in Quito and Mejía, particularly in Amaguaña (35 km NNW), La Armenia, Quitumbe (41 km NNW), Conocoto (41 km N), Guamaní (42 km NNW), La Ecuatoriana (44 km NNW), Turubamba (43 km NNW), Chillogallo (47 km NNW), La Magdalena (48 km NNW), Machachi, Tambillo (32 km NNW), Alóag (28 km NW), Cutuglahua (35 km NNW), Uyumbicho (30 km NNW), Aloasí (24 km NW), and El Chaupi (24 km WNW) during 2-4 February. On the morning of 3 February continuous gas-and-ash emissions were reported, resulting in slight ashfall in Tambillo, Chillogallo, and La Armenia 2 and later that same day in Guamaní, Turubamba, Chillogallo, La Ecuatoriana, Quitumbe, Tambillo, Machachi, Aloasí, Aloag (28 km NW), and Conocoto.

Figure 25. Webcam image of a gray ash plume rising above Cotopaxi on 2 February 2023. Courtesy of IG-EPN (IG Al Instante Informativo VOLCÁN COTOPAXI No 2023-023).

During 9-10, 12-19, and 26-27 February the Washington VAAC reported that gas-and-ash emissions rose 200-1,400 m above the crater and drifted W, NW, S, and SE. At 1800 on 9 February an ash plume rose 2 km above the summit and drifted W according to IG-EPN. Slight ashfall was reported in Tambillo. During the night and early morning of 11-12 February gas-and-ash emissions rose less than 500 m above the crater and drifted SW; there were some reports of light ashfall in El Chasqui (17 km W), Mulaló (19 km SW) and San Juan de Pastocalle (20 km WSW). Around 0810 the BREF seismic station recorded an increase in tremor energy, which is associated with ash emissions. During 13-14 February several gas-and-ash emissions rose as high as 1 km and drifted W and SW, resulting in minor ashfall in Mulaló, San Agustín (11 km W), Ticatilín (15 km WSW), San Ramón (17 km SW), Control Caspi (20 km WSW), and Pastocalle (22 km W). Starting around 0630 on 15 February ashfall was observed in Mulaló 2, San Ramón (17 km SW), Ticatilín (15 km SW), and San Agustín del Callo (18 km WSW). According to the Network of Volcanic Observers (ROVE), ashfall was reported in Latacunga canton (18 km WSW) during the night of 15 February. On 16 February at 0600 ash emissions rose 1 km above the summit and drifted E, based on surveillance cameras (figure 26). Starting at 0230 on 18 February IG reported an increase in seismic signals that were associated with ash emissions; by 0650 satellite images showed an ash plume that rose 800 m above the summit and drifted SE. Around 0030 on 19 February an ash plume rose 1.1 km above the crater and drifted S. During the morning of 27 February gas-and-ash emissions rose 1 km above the summit and drifted SE; light ashfall was reported in Pichincha en Rumiñahui (61 km N), Rumipamba Vallecito (55 km N), Conocoto, Pedregal (60 km N), Guamaní, Quitumbe, La Ecuatoriana, Chillogallo, Urubamba (Santo Tomas, 40 km NNW), La Magdalena (Barrio Nuevo, Villaflora), and San Bartolo. On 28 February at 1430 surveillance cameras showed an ash plume that rose 500 m above the summit and drifted SW.

Figure 26. Webcam image of an ash emission rising 1 km above the summit of Cotopaxi and drifting E on 16 February 2023. Photo has been color corrected. Courtesy of IG-EPN (IG Al Instante Informativo VOLCÁN COTOPAXI No 2023-031).

Ash emissions continued during March, rising 1-1.5 km above the summit, and drifting in different directions. Seismicity consisted of daily LP and near-daily TR events and less frequent VT and VLP events. Intermittent gas-and-steam emissions rose 100-1,500 m and drifted in different directions. According to the Washington VAAC ash emissions rose 500-2,000 m above the summit and drifted in multiple directions during 28 February to 3 March, 4-5, 18-19, and 24-29 March. Light ashfall was reported in Mulaló during 28 February to 1 March and 5-6 March. On 19 March at 0900 surveillance cameras captured an ash emission that rose 1 km above the summit and drifted E (figure 27). A thin ash cloud was observed in GOES-16 satellite images rising 500 m above the summit and drifted SW on 25 March. Around 0310 on 28 March a GOES-16 satellite image showed an ash cloud rising 1 km above the summit and drifted N. As a result, light ashfall was observed in Machachi and on the N flank of the volcano and in El Chasqui, Latacunga, and the S flank of the volcano during 28-29 March.

Figure 27. Webcam image of an ash emission rising 1 km above the summit of Cotopaxi and drifting E on 19 March 2023. Photo has been color corrected. Courtesy of IG-EPN (IG Al Instante Informativo VOLCÁN COTOPAXI No 2023-036).

Gas-and-steam emissions continued during April, rising 200-1,500 m above the crater, and drifting in different directions. Seismicity consisted of daily LP and near-daily TR events and less frequent VT events. Gas-and-ash emissions rose 200-1,500 m above the crater and drifted W, SW, E, and SE. During 4-6 and 9-10 April the Washington VAAC reported that ash emissions rose 200-1,100 m above the crater and drifted SW, SE, and E. A lahar occurred on the upper W flank on 5 April due to moderate rainfall. Light ashfall was reported in Mulaló and San Agustín during 5-6 April. On 8 April at 0900 a surveillance camera showed an ash plume that rose 500 m above the summit and drifted E and on 10 April at 0500 an ash plume rose 1.1 km above the summit and drifted W. During the night of 11 April, a small, secondary lahar was descended the Agualongo drainage. Another lahar was recorded during 12-13 April, though it was not observed due to cloud cover. On 15 April a small lahar was observed on the NW flank. The BNAS seismic station recorded an increase in seismic signal at 1600 on 22 April that corresponded to a small lahar, which descended the Cutzulao/Agualongo drainages. During 23-24 April the Washington VAAC reported ash emissions that rose as high as 2,028 m above the summit and drifted NE. According to IG-EPN surveillance camera showed an ash plume rising 3 km above the summit and drifted NE at 0953 (figure 28). During 24-25 and 28-30 April ash emissions rose 200-2,000 m above the crater and drifted NE and W. According to ROVE, ashfall was reported in the S area of the national park around 1805.

Figure 28. Webcam image of an ash emission rising 3 km above the summit of Cotopaxi and drifting NE on 24 April 2023. Photo has been color corrected. Courtesy of IG-EPN (INFORME DIARIO DEL VOLCAN COTOPAXI No. 2023-114).

Similar activity continued during May. Seismicity was characterized by daily LP and near-daily TR events and occasional VT events. Gas-and-steam emissions rose 200-1,500 m above the summit and drifted in different directions. Gas-and-ash emissions rose 200-1,400 m above the crater and drifted W, SW, NW, and SE. On 1 May at 0130 and on 8 May the BNAS seismic station recorded high-frequency seismic signals that corresponded to small lahars that remained within the Cotopaxi National Park. An ash plume was observed in a GOES-16 satellite image around 1100 that rose 1.5 km above the summit and drifted WNW, reaching Manabí on 4 May. Light ashfall was reported in Mulaló during 6-7 May. At 0600 on 7 May an ash plume rose 500-1,100 m above the summit and drifted W and SW. Ashfall was reported in Mulaló. The Washington VAAC reported that ash emissions rose 800 m above the summit and drifted NW during 7-8 May. During 9-10 May a moderate ash plume rose 2-3 km above the summit and drifted SW, N, and NE. Ashfall was reported in San Joaquín (SW) and San Agustín de Callo (SW). Another ash plume rose 2 km above the summit and drifted SE at 0749 on 12 May (figure 29). Intermittent ash plumes rose 1-3 km above the summit starting at 0510 on 18 May and drifted NE. As a result, light ashfall was reported in Machachi. Continuous ash emissions were recorded during the morning of 26 May, rising 1.5-2 km above the crater and drifting W and NW. Ash emissions rose as high as 1.1 km above the summit and drifted W and SW during 29-30 May according to the Washington VAAC. At 0600 surveillance cameras and satellite images showed an ash plume rising 1.1 km above the crater and drifted as far as 40 km W. Ashfall was reported in Pastocalle. During 30-31 May an ash emission rose 500 m above the summit and drifted W, based on a Washington VAAC report.

Figure 29. Webcam image showing an ash plume rising as high as 2 km above the summit of Cotopaxi on 12 May 2023. Courtesy of IG-EPN (IG Al Instante Informativo VOLCÁN COTOPAXI No 2023-048).

During June, gas-and-steam emissions rose 100-500 m above the summit and seismicity continued with daily LP and near-daily TR events and less frequent VT events. Gas-and-ash emissions rose 100-1,000 m above the summit and drifted in several directions. Light ashfall was reported in San Agustín de Callo and San Ramón during 1-2 June. Ash emissions rose 1 km above the summit and drifted SW during 1746-2000 on 3 June. According to the Washington VAAC, ash emissions rose 800 m above the summit and drifted SW during 4-5 June. A seismic signal was recorded on the NW flank on 8 June that IG reported was possibly associated with a small lahar. Around 0900 on 21 June an ash cloud rose less than 500 m above the summit and drifted over the W and SW flanks due to strong winds; light ashfall was reported in the Cotopaxi National Park. Starting around 1400 on 27 June through 28 June a high-frequency tremor signal was recorded at the BREF and BNAS seismic stations associated with small lahar signals moving through the Agualongo Creek; the flows were limited to the Cotopaxi National Park. Around 1300 on 29 June two seismic signals relating to small lahars were recorded on the W flank of the volcano moving down the Agualongo Creek.

Activity during July was relatively low and was mainly characterized by gas-and-steam emissions rising 100-1,800 m above the summit and seismicity consisting of daily LP and near-daily TR events and less frequent VT events. During the afternoon of 1 July, a high-frequency seismic signal was recorded on the NW flank relating to a small lahar. Gas-and-ash emissions rose 100-1,100 m above the summit and drifted SW and W. The Washington VAAC reported an ash emission that rose 500 m above the summit and drifted SW during 2-3 July. Light ashfall was reported in Mulaló, including Rosal, Ticatilín, San Agustín del Callo, San Ramón, and Rumipamba de Villacis. On 5 July around 0700 an ash plume rose 600 m above the summit and drifted SW, based on a GOES-16 satellite images. According to IG-EPN cameras and ROVE reports, remobilized ash was observed on the W flank resulting from strong winds during 18-19 July.

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN), Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); 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).

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

October 1975 (NSEB 01:01) Cite this Report

Vapor plumes rising above the crater in September and October

On 23 July 1975 small grayish puffs of smoke were observed emanating from the crater. In mid-September a 300-m vapor plume rose above the crater, and a small earthquake shook the volcano on 24 September. In mid-October vapor plumes were reported to be increasing in volume and frequency. Cotopaxi last erupted in 1944.

Information Contacts: M. Hall, Escuela Politécnica, Quito.

November 1975 (NSEB 01:02) Cite this Report

Continued vapor emanations in November

The activity continued during November. The amount of vapor was about the same as in October, but daily cloud activity was observed. There was increased fumarolic activity on the W side, just below the crater. Earthquakes were felt in the town of Mulaló around 0000, 11 November, and at 0431, 12 November, and again on 14 November. Three portable seismographs were placed around the volcano, and were gathering valuable seismic data by the end of the month.

Information Contacts: M. Hall, Escuela Politécnica, Quito.

December 1975 (NSEB 01:03) Cite this Report

Activity declines

Activity declined during December. The crater steam clouds of previous months were observed only once, on 12 December, since 28 November. Seismic activity was not strong and focal depths were at least 20 km below the volcano. Relevelling of dry tilt stations installed 7 November showed no inflation of the volcano. However, a USGS team including Donal Mullineaux and C. Dan Miller reached Ecuador in late December to assess volcanic hazards and assist the local authorities in contingency planning.

Information Contacts: M. Hall, Escuela Politécnica, Quito.

December 2002 (BGVN 27:12) Cite this Report

First anomalous seismicity since 1975 begins in October 2001

The last Cotopaxi report (SEAN 01:03) described a decline in activity during December 1975. Beginning in October 2001, anomalous seismic activity was registered. Seismicity increased further during November 2001-January 2002, and at times was up to seven times the normal level (tables 1 and 2). During this period, other seismic signals were registered that were distinct from those during the 13 previous years of monitoring, including: tornillos, explosion events, bands of harmonic tremor sometimes lasting a few minutes, and deep, high-energy long-period (LP) events registered away from the volcano (at the Antisana and Guagua Pichincha stations). Seismic observations and statistics were compiled using station "VCl," located ~4 km NE of the volcano. Earthquake locations were determined using records from the seven seismic stations on different flanks of Cotopaxi, and for higher-energy events with stations of the National network.

Table 1. Monthly seismicity at Cotopaxi during 2001-2002. Data includes Total and Daily averages for long-period (LP) events, hybrid events, volcano-tectonic (VT) events, tornillo events, and all earthquakes. Courtesy IG.

Table 2. Comparison of average seismicity at Cotopaxi during 2001 and 2002. Courtesy IG.

On 5 and 29 January 2002, two seismic clusters lasted an average of 2 hours and were composed mainly of LP and VT earthquakes. Most of the earthquakes were located at depths of 1-10 km beneath the summit. On 5 and 13 January small fumaroles were reported in the crater, and visible defrosting occurred on the upper E flank. A visit to the summit on 13 January revealed increased fumarolic activity compared to previous months. On 19 and 20 January observers reported gray plumes rising as high as 1,000 m.

During February and March activity diminished, and no seismic clusters were registered. Most of the earthquakes were located 1-10 km beneath the volcano. On 5 February roaring noises were heard from Mulaló and the refuges located on the flanks of the volcano. Strong fumarolic activity was also reported. On 6 February steam plumes rose ~300 m above the summit. On 27 February a small steam plume was reported exiting from the NW side of the crater. On 7 and 10 March small steam plumes originated from the W side of the crater. On 28 March harmonic tremor lasted for ~10 minutes.

Activity remained low during April-June. On 17 April a band of harmonic tremor lasted ~6 minutes with a maximum frequency of 4.3 Hz. During the first days of April small steam plumes were reported. During May LP earthquakes lasted up to a minute and saturated the seismometer for several seconds. On 20 May a seismic cluster of LP earthquakes lasted ~2 hours. On 8 and 14 May a white steam plume from the NE side of the volcano reached up to 200 m high. During June VT events mostly occurred ~10 km N of the crater. On 30 June a band of harmonic tremor lasted ~7 minutes with a maximum frequency of 1.7-5.2 Hz. Visits to the summit on 1 and 2 June revealed that fumarolic activity had diminished ~40% since January.

During July seismicity was at a moderate level with respect to the rest of 2002. During the first days of the month a series of LP events were registered that were large enough to be detected at distant stations, such as Antisana and Guagua Pichincha. The earthquakes had maximum frequencies of ~2.1 Hz and were generally 1-2 km beneath the summit. However, some events were located at depths of ~10 km. On 18 July at 2000 a band of low-frequency tremor lasted ~4 minutes. About 5 hours later a seismic cluster began that lasted for ~8 hours. The cluster consisted of ~110 total events, mostly hybrid (HB) and volcano-tectonic (VT). The earthquakes were located 1-4 km beneath the summit, and 2 LP events were located ~10 km deep.

Visitors to the summit on 6 July reported fumarolic activity in the zone of Yanasacha, a slight sulfur smell on the NE side, and noise generated by an avalanche on the E side. At the end of July reports indicated defrosting in the W zone. During August moderate seismicity was dominated by LP events at a depth of ~10 km.

Seismicity was again high in September 2002. A small cluster of VT earthquakes on 15 September lasted ~7 hours. During the first days of the month a visit to the crater revealed new fumaroles in the E and S zones. Defrosting continued in the W zone and left 40% of the W wall open.

During October seismic activity was low but the number of hybrid events increased compared to the previous months. Tectonic events were registered in the S and N zones up to ~7 km from the summit. Deep LP events decreased by ~50% compared to previous months.

Seismicity remained low during November and December. Less than 10% of VT events were registered in the N sector. No fumarolic or other surface activity was observed. During December seismic events were located 1-7 km beneath the summit. On 7 December people in Yanahurco reported dark brown plumes rising from the crater.

Seismicity since 1989 clearly shows an increase in recent months (figure 1). The 2001 seismic events were registered at 1-10 km beneath the volcano, but ~90% occurred at 2-4 km and showed little migration. The 2002 activity was variable, from a high of 966 events in January to a low of 420 events in April. Mostly LP events occurred with some VT events during the first half of the year, and later mostly LP events with hybrids during the second half of the year. On the basis of 2002 seismic activity, a new injection of magma did not occur, and the anomalies in July and September were the result of the movement of gas from magma intrusion that occurred during the last months of 2001.

Figure 1. Graph of the total registered monthly events at Cotopaxi during 1989-2002. The activity increased beginning in November 2001 and has since remained above background levels. Courtesy of IG.

Information Contacts: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador.

November 2003 (BGVN 28:11) Cite this Report

Low seismicity and emission signals January-May 2003; March earthquake clusters

This report contains details of seismicity at Cotopaxi during January through 2 May 2003. The seismicity was generally low (averaging ~20 earthquakes per day), as it has been since 24 November 2001. Despite the low seismicity, during January seismic signals suggestive of emissions registered, although these lacked visual confirmations at the volcano. Moreover, a cluster composed of a variety of kinds of shallow earthquakes took place in mid-March. This was the first such cluster since 19 July 2002.

Activity during January-February 2003. Seismicity was generally low in January 2003 and located earthquakes commonly had focal depths down to 5 km below the summit. During the first week of January one volcano-tectonic (VT) event occurred N of the volcano. Around this time the rate of energy release was very low and no unusual observations were reported. Seismicity decreased after the first week of January, although some long-period (LP) events occurred, including one of high frequency (10 Hz) on 9 January that was followed immediately by another with a slowly decaying coda or tail (a so-called "tornillo" event, with a dominant frequency of 2.7 Hz). Two LP events were located at depths of 1 km. The rate of energy release remained very low, with some peaks on 8 January. Seismicity stayed low through the next week; some hybrid and LP events did occur. Some signals characteristic of emissions were received, although these were not visually confirmed.

During 20-26 January the number of hybrid events increased slightly, to above average. Emission signals were again received, similar to the previous week. No LP earthquakes were recorded this week, but a small group of earthquakes were located at the headwaters of the Pita river. Events such as these were also noted in November 2001. During the last week of January, seismicity remained low, on a par with activity seen since 24 November 2001. However, the low number of events registered or located was partly because arrivals were not clear at many stations.

Seismicity remained low in February, particularly for the first week. During 10-16 February it rose slightly due to larger numbers of hybrid events. No other changes in the volcano were noted. Although the third week of February brought no important variations in seismicity, beginning in late February LP events dominated the record. Still, the number of LP event stayed below the 2002 average.

Activity during March-April 2003. Although low seismicity generally prevailed throughout this interval, there was some variations in the abundance of earthquake types and a mid-March cluster of earthquakes occurred. During early March hybrid earthquakes increased to slightly higher than the 2002 average; in addition another LP-type tornillo was recorded on 6 March. On 7 March LP earthquakes were common.

On 16 March a cluster of hybrid, VT, and LP earthquakes was located 1-3 km below the volcano. Following eight months of low seismicity (averaging ~20 events per day), this was the first seismic swarm registered at Cotopaxi since 19 July 2002. However, the energy released per number of events was similar to earlier activity.

Seismicity increased after 16 March. Clusters similar to that of the 16th continued, but with lower magnitudes. By the beginning of April seismicity decreased to within the base level, although on 4, 7, and 8 April VT events were recorded to the S and SE, approximately 3 km below the summit. No significant changes were noted at the volcano, although the usual smell of sulfur was noted on a visit to the summit. During 14-20 April, the number of LP events decreased from the previous week, but VT events of M 2.5-M 3.4 continued to the N. VT events persisted through the rest of April, particularly in late April, which on 23 April included an M 3.6 event. VT events occurred on the N, NE, and S sides of the volcano up to 15 km from the summit at depths between 3 and 15 km. The VT events were interpreted as related to rock fracturing.

On the morning of 2 May a VT event registered on the S flank, located ~3 km deep. It was M 3.2, moderate for Cotopaxi. Later that day an event registered at the seismic stations at Cotopaxi, Antisana, and Guagua Pichincha. This event had a duration of 180 seconds and was made up of an LP earthquake followed by a tremor-like signal with a duration of 150 seconds that was of low frequency (1.6 Hz).

Information Contacts: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/).

December 2003 (BGVN 28:12) Cite this Report

During May-December 2003 seismicity moderate, degassing and inflation variable

This report contains details of seismicity at Cotopaxi during May through December 2003. In general, seismicity was low and within normal levels, occasionally punctuated by increased activity. Fumarolic and inflationary activity varied throughout the period.

Seismicity during the first week of May was characterized by a high number of fracture-related volcano-tectonic events in the N, NE, and S zones, up to 15 km from the summit. These events were located at depths between 3 and 15 km below the summit. On 2 May at 0949 a volcano-tectonic event on the S flank occurred at ~ 3 km depth. Based on the coda, the event was calculated as M 3.2, a value considered moderate at this volcano. At 1918 on 2 May a long-period event was recorded at the Cotopaxi, Antisana, and Guagua Pichincha seismic stations. It lasted about 180 seconds. The earthquake was followed by a low-frequency (1.6 Hz) tremor signal lasting about 150 seconds.

Between 2 and 4 May deflation was recorded, with slight variations. On 2 May staff at the Refuge felt earthquakes. On 3 May the staff saw steam plumes at heights of 400-800 m above the crater, which blew W. On 3 and 4 May observations were made at the Refuge and the summit. Staff smelled sulfur halfway to the summit; and found new fumaroles in the Yanasacha area. On 4 May these fumaroles generated white steam plumes up to 50 m above the summit. Fumarole temperatures were 29-31°C.

A tectonic earthquake was recorded on 8 May, but although tremor episodes increased, volcano-tectonic earthquakes were fewer during 5-11 May than the previous week. Seismicity continued to drop during the week of 12-18 May. Although some low-amplitude tremor occurred during that interval, activity was dominated by long-period earthquakes. Earthquakes increased slightly the following week, but seismicity remained lower than average for the year. Low-frequency tremor lasting under 10 minutes was recorded on 23 May; tectonic activity on 24 May occurred in the zone of Saquisili and was determined to be unrelated to Cotopaxi. During the final week of May, long-period events and tremor signals increased slightly but seismicity continued to remain within the normal parameters established as of November 2001, when Cotopaxi entered a period of unusual seismic and fumarolic activity.

Activity remained generally constant through June, with episodes of harmonic tremor increasing slightly between 9 and 15 June and again on 23 June. White steam plumes reached 300 m high on 4 June, but later they were under 100 m high. At the end of June there was a slight tendency toward deflation; tremor events increased slightly and usually had fundamental frequencies of ~ 1.7 Hz.

Between 7 and 13 July the number of long-period events increased, as did the number of hybrid events. However, tremor decreased, and the average number of earthquakes per day (8) was lower than in recent periods of increased activity. The average number of earthquakes per day decreased again the following week. Notable tremor occurred on 20 July, with episodes lasting between 80 and 125 seconds and reduced displacement varying from 0.5 to 11 cm². During the week of 21-27 July activity increased slightly, from 6.6 to 8.3 events per day, but in general seismic data indicated a state of low activity during July.

In early August seismicity rose to an average of 20 events per day, and tremor signals increased, especially on 8-10 August. However, the released energy remained low throughout August. Earthquakes registered that month were generally small, and tremor signals were constant except for two periods of harmonic tremor on 28 August.

Although seismicity remained low in early September, on 6 September instruments registered a low-frequency (0.9 Hz), low-amplitude tremor lasting more than 3 hours. On 18 September a cluster of earthquakes (characterized by long-period events and hybrid events) began around 1300 and lasted ~ 4 hours. A second cluster occurred the next day, lasting ~ 6 hours. The earthquakes associated with these clusters were located between 1 and 4 km below the summit. Fumarolic activity was normal for most of September, although a gas discharge was reported on 21 September. After 21 September seismicity returned to normal levels, and continued to decrease through the following week.

Seismicity generally remained low for the next few months. Volcano-tectonic earthquakes and tremor increased slightly during 13-19 October. Three distinct episodes of tremor on 15, 17, and 18 October consisted of similar events with dominant frequencies of 0.8-0.9 Hz. Seismicity into November remained low, with no significant episodes of tremor and only small events.

By mid-December seismicity increased and although activity remained within normal levels, the occurrence of high-frequency tremor was noteworthy. Also through mid-December, a slight odor of sulfur was reported, as well as occasional columns of steam no higher than 300 m.

Correction: A brown plume mentioned on 7 December 2002 (BGVN 27:12) might be misinterpreted as evidence of an ash-bearing emission. Gorki Ruiz, a colleague of Pete and Patty Hall, clarified events and interpretations from that date. He interviewed guards at the Cotopaxi refugio, who stated that neither they nor others at the refugio that day had observed emissions. They discounted observations of ash emissions and noted that although fumarolic plumes frequently reach 300 m above the summit, no phreatic explosions had occurred. That time interval was also one of low seismicity.

Information Contacts: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/).

November 2004 (BGVN 29:11) Cite this Report

Seismically quiet in January-April 2004; planning for emergency water supplies

Seismicity at Cotopaxi during December 2003 through December 2004 yielded averages that generally remained within normal levels (table 3). Steam emissions continued, and sulfurous odors were occasionally reported. A plot of total seismicity each week during 2001-July 2004 portrayed numerous peaks and valleys in the range 50-200 events per week. Occasional excursions took the weekly totals to several hundred events in late 2001 and early 2002 (peaking at over 700 events per week during mid-October 2001). The 2004 data lacked such dramatic excursions.

Table 3. Annual summaries showing typical daily averages of various kinds of seismicity at Cotopaxi during 2001-2004. Courtesy of IG (shown on their website in the January 2005 report).

Planning for emergency water supplies. Although seismicity and other monitored parameters were moderate to low during most of 2003 (BGVN28:11 and 28:12), local authorities worked on a contingency plan for emergency drinking water in the event of a crisis at Cotopaxi.

The Quito metropolitan sanitation and drinking water company (EMAAP-Q) prepared a contingency plan for residents around Cotopaxi. The challenge was to provide for sufficient amounts of potable and sanitation water for some half a million people in the event of an eruption that contaminates their normal water supplies. This contingency plan was drawn up using experience gained from the operational emergency plan used to recover from the eruption in 1998-99 and the Reventador eruption in 2002.

During the Guagua Pichincha eruption, pyroclastic material impacted Quito, and ash fell into the water treatment plants and threatened the water supply systems. EMAAP-Q developed an operational and emergency plan. The plan was tested in 1999 when the volcano had two major eruptions that heat dropped ash on Quito and its infrastructure.

Information Contacts: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/).

April 2016 (BGVN 41:04) Cite this Report

First eruption since the 1940's begins 14 August 2015

Historically, Cotopaxi has been an active, dangerous volcano, although it had been mostly quiet for the previous 70 years before beginning a new eruptive period during August-November 2015 (figure 2). Increased seismic activity, deformation, ash plumes and ashfall, SO₂ emissions, fumarolic activity, and lahars were all observed by geoscientists from the Geophysical Institute of Ecuador (IG) during the 2015 eruptive phase; information in this report comes from IG unless otherwise noted.

Figure 2. Cotopaxi volcano, 21 September 2015. View is towards the south; the plume is rising 1.5 km above the summit and drifting WSW. Photo by B. Bernard, courtesy of IG.

The first phreatic explosions took place on 14 August 2015 and spread ash across the region up to 48 km NW, followed by a pyroclastic flow on the W flank the next morning. The first eruptive phase with numerous explosions and high seismicity continued through mid-September. A shorter explosive episode occurred in October, and the last significant ashfall was in late November. The Cotopaxi National Park was reopened to visitors in late December, as both seismic and surficial activity had declined significantly. The last ash emission was reported on 24 January 2016. The first rain-induced lahar occurred in late August, and numerous others were recorded in November 2015 and February 2016.

Pre-eruption activity. During 2002 and 2003, observations of increased seismicity included a large number of fracture-related volcano-tectonic (VT) events located between 1 and 15 km below the summit accompanied by fumarolic activity and occasional steam plumes (BGVN 28:12). In spite of this heightened activity, no ash-bearing emissions were detected. From July through early September 2005, the number of VT earthquakes within a 15-km radius of the summit again increased from background levels, but none were large enough to be felt nearby. These earthquakes had a maximum magnitude of M 4.4, and ranged from 3 to 10 km below the summit. IG scientists interpreted this increased seismic activity, characterized by long-period (LP) and hybrid (HB) earthquakes, to be related to fluid mobilization. The level and magnitude of activity was similar to that which occurred during the November 2001-January 2002 period. This increase in seismicity was also shown to correlate with elevated fumarolic activity as witnessed from a video camera installed on the NW edge of the crater. Again, no ash-bearing emissions were detected.

From late 2005 through early 2015 Cotopaxi was closely monitored for seismic, thermal, fumarolic, and lahar activities. These activities remained within normal background levels until April 2015 (figure 3). Background levels of total seismic events had been in the range of 10-25 per day since 1996. They began to increase noticeably at the end of April, and reached a monthly total of 3,000 in May. The activity was located in the N and NE parts of the cone, at two different depths, 3 and 14 km below the summit.

Figure 3. Rate of very long period (VLP) seismic events, associated with the movement of magma inside Cotopaxi, between January and the end of May 2015, showing an increase in the daily number of events beginning in late April. Courtesy of IG.

Sulfur dioxide (SO₂) emissions also increased from background levels of ~ 500 tons/day to 2,500-3,000 tons/day in late May and early June; climbers indicated intense sulfur dioxide odor on the N part of the cone during 22-23 May. Crater fumarolic activity also increased, with plumes sometimes visible from Quito (55 km N). Low-energy, pulsating gas emissions began on 10 June, and Cotopaxi National Park staff reported an increase of water flow in streams on the NE flank, suggesting increased heat flow around the volcano.

New eruption begins on 14 August 2015. An earthquake swarm beginning in the late afternoon of 13 August 2015 was followed in the early morning of 14 August by two phreatic explosions (at 0402 and 0407). A third explosion occurred at 1027, and two additional smaller explosions were recorded later in the day at 1345 and 1429 (figure 4).

Figure 4. Seismograph from 14 August 2015, station BREF Courtesy of IG (Cotopaxi 2015 Special Report No. 7).

These eruptions were witnessed by many residents in the surrounding towns and from the Metropolitan District of Quito. The Washington Volcanic Ash Advisory Center (VAAC) reported plumes from the first two explosions rising to altitudes of 12.2-13.7 km, and IG reported that the plume from the 1027 eruption rose to an altitude of 17.9 km (58,700 ft). At 0715 on 15 August a pyroclastic flow descended the W flank, sending another ash plume to at least 15.2 km, according to Washington VAAC. The lower portions of the first two plumes drifted E and the higher parts drifted SE. The plume from the third explosion drifted NW and E. Ash from the pyroclastic event drifted 17 km W, 20 km NNW, and 8 km SE as reported by Washington VAAC. According to IG special reports, ashfall was reported in areas N and NW of the volcano (figure 5) as far as 48 km away in the Machachi region, Amaguaña, Bowling, Tambillo, and S of Quito. Parts of the Cotopaxi National Park were closed to visitors. During 16-18 August, ash emissions of variable height up to 1 km continued, drifting W.

Figure 5. Preliminary map of the ashfall deposits associated with the eruption of Cotopaxi on 14 August 2015. Courtesy of IG (Cotopaxi 2015 Special Report No. 9).

The ashfall grainsize was sub-millimeter, consisting of granules, rock fragments, and mineral crystals typical of the andesitic rock of Cotopaxi. All fragments from the first eruptions appeared to be pre-existing rock, some altered, with no juvenile particles from new magma observed.

During the post-eruption overflight on 18 August, IG scientists observed variable amounts of ash and steam rising more than 100 m above the crater before descending the W flank. Significant amounts of ash were deposited on the flanks in an area from the N to the SW. New cracks on the top of some glaciers were noted, especially on the E and NE flanks, and possible new tephra deposits on the N flank were observed. Thermal images did not reveal hot material on the flank and emissions prevented measurements inside the crater. For the next week after the initial explosions, steam-and-gas emissions were occasionally observed (figure 6), with plumes rising less than 2 km above the crater.

Figure 6. Photo of Cotopaxi taken on 20 August 2015 showing a small explosion at 0740 as viewed from the Sector 2 Bridge along the General Rumiñahui Highway, 47 km N. Note the recent ash deposits covering the glacier near the top of the mountain. Photo by Camilo Zapata , courtesy of IG (Cotopaxi 2015 Special Report No. 9).

On 22 August at 0426 the network detected an increase in the seismic amplitude, and steam-and-ash plume emissions rose to more than 3 km above the summit. More tremors at 2141 were accompanied by the onset of continuous ash emissions which rose less than 1 km above the crater, drifting SW. Rangers confirmed ashfall at the entrance of Cotopaxi National Park. The following morning IG staff found 2-mm-thick ash deposits that had accumulated during an 18-hour period. Ash deposits were also noted on 24 and 25 August in most of the N parts of Latacunga valley located about 18 km WSW of the mountain, as well as the S hills of Romerillos 15 km NW, Machachi (24 km NW), El Chaupi (24 km WNW), and Tambillo (32 km NNW).

Regular overflights were conducted by IG after the initial eruption. In September 2015 several new cracks in the glaciers were noted, and volcanic blocks had been deposited on the N and S flanks of the crater. The circular glacier at the top of the crater had significantly decreased in size and developed large fractures. Glacial melting on the upper flanks had also accelerated; streams of meltwater were present on the N flank.

By late September, the glacier inside the crater had almost disappeared. Several areas of landslide deposits inside and outside the crater were also noted. Yellowish-green deposits from increased fumarolic activity were most apparent on the S, SE, and E flanks. Rapid melting from the glacier was evident on the upper E flank which resulted in material falling onto the lower part of the glacier. Seismicity, deformation, fumaroles, ash plumes and ash fall, SO₂ emissions, and lahar activity all continued for several months as discussed below, gradually tapering off toward the end of 2015.

Seismic activity. Seismic data during the August through December 2015 period correlated closely with the rise and fall of eruptive activity. The measurements of the average seismic amplitudes (RSAM) for the volcano (figure 7) reveal the dramatic increase in total seismic energy during the main eruptive phase between mid-August and early September.

Figure 7. Measurement of daily average seismic amplitude (RSAM) for Cotopaxi from 1 January 2015 through 10 October 2015. The solid and pink-dashed lines represent previous background averages for RSAM. Courtesy of IG (Cotopaxi 2015 Special Report No. 20).

Numbers of tremor signals indicating emissions (figure 8) showed a dramatic increase beginning on 14 August, and remained elevated through the initial eruptive cycle ending in early September. A second episode of emission activity in mid-October had fewer events, but the number was still higher than background levels. Seismic activity from emissions tapered off substantially after mid-October.

Figure 8. Plot of number of daily seismic tremor events indicating emissions at Cotopaxi, 1 August through 19 October 2015. Courtesy of IG (Cotopaxi 2015 Special Report No. 20).

The number of daily volcano-tectonic (VT), or fracture-related earthquakes, located immediately beneath the crater to a depth of 12 km, increased starting in mid-August, to a maximum of over 200 events on 20 September before declining below 100 in early October. In September, the higher daily number of VT signals 3-12 km deep aligned with the conduit were interpreted by IG scientists as movement in a magma reservoir. Elevated numbers of VT events continued into February 2016, when they dropped back close to background levels (figure 9).

Figure 9. Plot showing number of daily volcano-tectonic (VT) (top), explosive (middle), and hybrid-type (HB) (bottom) seismic events at Cotopaxi between 1 January 2015 and 15 February 2016 (G. Viracucha). Courtesy of IG (Cotopaxi 2016 Special Report No. 4).

Deformation. As part of the activities conducted jointly by the French IRD (Institut de Recherché pour le Developpement) and IG, Jean Mathieu Nocquet analyzed the deformation data from the volcano (figure 10). The accumulated movement of the GPS points between 1 January and 1 October 2015 is shown with values corrected for drift due to tectonic movement and seasonal effects based on data from 2010-2015. The strain level was low but significant compared to other deformation events prior to eruptions. The rate of deformation began accelerating in April 2015. The maximum total movement during this time was 1 cm at sites on the W flank. The average rate of deformation observed during this period was 3 mm per month.

Figure 10. Deformation observed between 1 January and 1 October 2015 at GPS stations after correcting values for tectonic movement and seasonal changes. The yellow arrows show horizontal movement of the terrain and red show vertical changes. Scales are shown in the lower left corner. Produced by Dr. Jean Mathieu Nocquet. Courtesy of IG (Cotopaxi 2015 Special Report No. 20).

Ash and steam emissions. Ashfall calculations reported by IG for 14 August through 25 September were 771,000 m³ and 9.46 × 108 kg. IG scientists estimated that the volume of the initial plume material ejected on 14 August was 56,000 m³. The largest volume of tephra was erupted during 22-28 August. The area most affected that week was west of the volcano between San Juan de Pastocalle (20 km WSW) and El Chaupi (24 km WNW). Based on Washington VAAC reports, IG noted on 26 August that the plume rose as high as 9 km. As a result of this, during the last week in August, areas over 200 km to the W, WSW and SW, and up to 95 km NW reported ashfall, millimeters thick in some areas. The mayor of Sigchos, in the Province of Cotopaxi, noted impacts on livestock, crops, and greenhouses (figure 11).

Figure 11. Farmland covered by a blanket of ash in Romerillos, 15 km NW of Cotopaxi, on 28 August 2015. Photo by P. Mothes. Courtesy of IG (Cotopaxi 2015 Special Report 13).

A news article from 14 September noted that area flights had been re-routed around Cotopaxi to avoid ash plumes; the most affected route was between Quito and Guayaquil. Based on information from Washington VAAC, during 4-11 September the ash clouds associated with activity at Cotopaxi affected a large area of Ecuador (figure 12).

Figure 12. Zones with potential ash cloud hazards associated with the activity of Cotopaxi during 4-11 September 2015. Courtesy of IG (Cotopaxi 2015 Special Report 16, using data from the Washington VAAC on a Google Earth base map).

The height of the ash clouds during 4-11 September decreased from over 6 km above the crater the previous week to less than 3 km, along with a decrease in the intensity of seismic activity. The speed of the ash clouds during this week varied between 3.9 and 14.1 meters per second (m/s) with the prevailing wind direction towards the west (between SW and W). Clouds of ash reached the W coast, drifting as far as 465 km on 9 September. There were at least two VAAC reports issued in October, but the volumes and masses of these eruptions were not calculated.

Significant ash-and-steam emissions continued from Cotopaxi until 23 November 2015 . The plumes generally rose no more than 2 or 2.5 km above the crater and usually drifted N or NW, and occasionally W or SW. Intermittent ashfall was reported from numerous towns through 18 November, generally from areas W and N of the volcano at distances up to 60 km. A summary of towns that reported ash and their distance and direction from Cotopaxi is shown in Table 4. The last significant ashfall reported by IG was on 23 November 2015. By December, the height of the steam plumes had decreased to consistently 1 km or less above the crater. No ashfall was reported in December inside the park or surrounding area, but traces of ash were observed covering parts of the glacier on the flanks of the volcano. At 1843 on 24 January a plume with low-to-moderate levels of ash rose 700 m above the crater and drifted W. The emission coincided with a hybrid earthquake. This was the last reported ash through March 2016.

Ash characteristics. Analysis of ash collected on 2 September showed that the greatest contribution of material was from pre-existing and altered rock. A week later, analysis of ash samples showed an increase in the proportion of juvenile fragments. A grain size analysis of ash samples indicated a very high proportion of extremely fine ash (30 to 75% less than 63 microns). Other than the ash recovered on the first day of the eruption (14 August ) which had a slightly larger size distribution, subsequent samples were consistently very fine to extremely fine in size. Component analysis performed with binocular and scanning electron microscopes clearly showed an evolution of the ash toward a decreased contribution of preexisting rock (lithic fragments with pyrite, grey scoria with vesicles filled with hydrothermal material, hydrothermal quartz) and an increased proportion of fresh magmatic material (free crystals, glass particles with few vesicles and a high percentage of microlites), especially after 28 August (figure 13).

Figure 13. Left: Binocular microscope photograph of low-vesicularity glass particles from 28 August, 2015 (photo: A. Proano); Right: SEM image of a semi-vesicular particle with abundant microlites (crystals

SO₂ emissions. SO₂ emissions at Cotopaxi began to rise above typical background levels of ~ 500 t/day in late April 2015, and were consistently above 2,500 t/d by late May (figure 14). Values measured by IG scientists peaked on 15 August, the day after the initial explosion, at 16,700 t/d, and declined to 5,490 by 20 August. Emissions on 14 August were also detected by the Ozone Monitoring Instrument (OMI) (figure 15). A total of 350,000 tons of SO₂ emissions was recorded in August 2015. Values spiked between 5,000 and 10,000 t/d numerous times between 14 August and late November 2015 before slowly decreasing to levels below 1000 t/d by January 2016, somewhat above background averages.

Figure 14. Plot of SO2 emissions (tons/day) at Cotopaxi from 1 January 2015 through 15 February 2016. Courtesy of IG (Cotopaxi 2016 Special Report 4, graph created by D. Sierra, IGEPN).

Area where SO2 gas emissions from Cotopaxi were detected by the OMI instrument on 14 August 2015. Red indicates higher concentrations. Courtesy NASA Goddard Space Flight Center (GSFC).

Thermal Monitoring. The first thermal images obtained after the explosions in August occurred during overflights on 18 and 26 August, and revealed a significant increase in the temperatures of emissions. The Maximum Apparent Temperature (MAT) was 150° C on 26 August at several different areas within the crater. By 3 September, thermal images revealed temperature increases in the S and E parts of the crater to 200° C. Bright areas at the summit were observed at night, possibly from hot block deposits. During the 9 September overflight infrared measurements indicated that temperatures had begun to decrease. This decrease continued until 29 September when new thermal anomalies on the upper parts of the outer crater were identified, likely from newly deposited material.

On a clear 2 October, IG geoscientists were able to measure and photograph the thermal anomalies identified and analyzed in previous flights (figure 16). The Maximum Apparent Temperature for the entire inner crater was 104.3° C. The MAT measured on the E flank of the volcano was 57.9° C, up from 51° C measured in the previous flight on 29 September. By 26 January, the thermal image data revealed a significant decline of the MAT to 51.9° C at the bottom of the inner crater.

Figure 16. Thermal image showing the highest value of MAT corresponding to the inner crater at 104.3° C. The MAT flank temperature is 57.9° C. Courtesy of IG (Cotopaxi 2015 Special Report 19; image by M. Almeida and S Vallejo, IG-EPN).

Lahars. The first lahar generated by the eruptions descended the W flank of Cotopaxi on 28 August, detected as a high-frequency seismic signal from two stations on the W flank (figure 17). It was a small flow remobilized by a light rain. IG technicians located the flow and determined that the flow rate did not exceed 10 m³/s, and that it remained near the top of the volcano.

Figure 17. A small lahar generated by minor rainfall descended the W flank of Cotopaxi on the afternoon of 28 August 2015. It was located far from any road or infrastructure. Courtesy of IG (Cotopaxi 2015 Special Report 13; photo by D. Andrade).

There were 58 total rainfall-induced lahars detected through March 2016, with the largest numbers occurring in November and February. They primarily descended the NW and W flanks of the volcano. Four small lahars from the melting glacier were reported in December. The largest lahar occurred at 1336 on 13 January in the Agualongo gorge (approximately 15 km. SW of the summit); a seismic station on Cotopaxi's W flank recorded an hour-long signal caused by the lahar. Preliminary results indicated that the peak flow rate was around 40 m³/s with a volume of about 50,000 m³. The lahar left deposits on the bridge that crosses the river (figure 18).

Figure 18. The System ECU-911 camera photographed the evidence left after the passage of a lahar across a bridge over the Agualongo gorge (approx. 15 km SW of the summit) on 13 January 2016. Courtesy of IG (Cotopaxi 2016 Special Report 2).

Post-eruption activity. In late December, after 119 days of closure due to volcanic activity, the Cotopaxi National Park reopened to visitors, as both seismic and surficial activity had declined significantly. By 29 January IG reported that surficial activity was characterized by minor steam emissions from the crater and sporadic gas emissions with minor amounts of ash (the last ash emission was reported on 24 January). Sulfur dioxide (SO₂) emissions were less than 1,000 tons per day (near pre-eruptive levels) by the end of January and seismicity had almost returned to baseline levels. Observations in late January 2016 indicated that the glaciers were continuing to move downslope with fractures and cracking, and continued glacial melting was observed as well.

Cotopaxi remained quiet through March 2016 with minor gas emissions rising 600 m above the crater and drifting N and NW. The glacier remained cracked, and glacial melting had decreased considerably.

Table 4. Towns and villages reporting ashfall from Cotopaxi, August-November 2014.

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional, Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); 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/); OMI (Ozone Monitoring Instrument), Global Sulfur Dioxide Monitoring, Atmospheric Chemistry & Dynamics, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); El Comercio (URL: http://www.elcomercio.com/actualidad/aerovia-actividad-cotopaxi-ecuador-negocios.html).

February 2023 (BGVN 48:02) Cite this Report

New eruption in October 2022; frequent ash emissions continuing through January 2023

Cotopaxi, located in Ecuador, is a steep-sided cone capped by nested summit craters, the largest of which is about 550 x 800 m. Pyroclastic flows have accompanied many explosive eruptions and lahars have frequently affected adjacent valleys. Strong eruptions took place in 1744, 1768, and 1877. The most recent eruption took place during August 2015 through January 2016 that was characterized by phreatic explosions, ash plumes, pyroclastic flows, lahars, and ash fall (BGVN 41:05). A new eruption began in October 2022; this report describes activity from October 2022 through January 2023, which included frequent ash plumes, ashfall, and rare lahars. Information comes from Ecuador's Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN), Servicio Nacional de Gestión de Riesgos y Emergencias (SNGRE), the Washington Volcanic Ash Advisory Center (VAAC), and various satellite data.

Activity has been relatively quiet since the last eruption in January 2016. According to the Washington VAAC, four ash clouds were detected on 4 July 2016, 23 January 2017, 15 July 2018, and 10 January 2020. However, no seismicity or ash deposits were associated with these events, so their cause is unknown and has not been attributed to eruptive activity.

During 21-31 October gas-and-steam emissions rose 200-1,000 m above the crater and seismicity was mainly characterized by daily small, long-period (LP) type earthquakes, volcano-tectonic (VT) events, and volcanic tremor (TR). On 21 October at 1944 seismic stations on the flanks began to record a volcanic tremor event. This earthquake signal lasted until 0040 on 22 October and was accompanied by gas-and-ash emissions that drifted NE; a moderate amount of ashfall was visible in the Refugio José Rivas, 2 km N of the summit crater, in addition to a sulfur odor (figure 19). The Washington VAAC reported a diffuse ash cloud seen in GOES-16 satellite imagery at 2200 on 21 October that rose to 1.7-2.3 km above the crater and drifted NE and W. A Sentinel-2 satellite image showed ash deposits on the N flank on 23 October (figure 20). Seismic stations on the N flank detected a signal related to small lahars during 1115-1300 on 23 October, which were generated due to a melted glacier underlying the ash deposit. An analysis of ash samples collected by IG scientists at the Refugio José Rivas showed that about 22% was juvenile material.

Figure 19. Photos of ash deposits from the eruption at Cotopaxi on 21 October 2022. Photos by Benjamin Bernard, IG-EPN (A), Cristian Rivera, ASEGUIM (B), and Marco Solís, IG-EPN (C). Courtesy of IG-EPN.

Figure 20. Sentinel-2 satellite image showing ashfall deposits on the N flank of Cotopaxi on 23 October 2022. Image uses “Natural color” rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

During November, frequent gas-and-steam emissions rose 200-2,800 above the crater and seismicity mostly consisted of daily small LP events and occasional TR and VT events. During 23-24 November ash emissions rose 200-1,000 m above the crater and drifted E, causing visible ash deposits on the NE flank. A new episode of tremor associated with gas-and-ash emissions was detected at approximately 0310 on 26 November that lasted for several hours (figure 21). An ash plume rose 2.2 km above the crater on 26 November and extended 60-85 km NNW. Ashfall was visible in various sectors in the Mejía, Rumiñahui, and Quito areas, including El Pedregal (60 km N), Tambillo (32 km NNW), Guamaní (42 km NNW), Amaguaña (33 km NNW), Chillogallo (44 km NNW), Quitumbe (41 km NNW), Solanda (46 km NNW), Lloa (48 km NNW), Conocoto (41 km N), Mercado Mayorista (45 km NNW), Villaflora (47 km NNW), and Rumipamba (55 km N); an estimated 7-20 x 10⁶ kg of ash were measured. On 27 November gas-and-steam emissions rose 2.8 km above the summit. The Washington VAAC reported during 26-27 November gas-and-ash emissions rose 1.7 km above the summit and drifted NW. Ash emissions rose 600 m above the crater and drifted E on 30 November. IG noted that light ashfall in Latacunga and on the S flank may have been due to remobilization of previously erupted ash.

Figure 21. Webcam image of a gas-and-ash plume rising above Cotopaxi on 26 November 2022. Photo has been color corrected. Courtesy of IG (INFORME DIARIO DEL VOLCAN COTOPAXI No. 2022-036).

Seismicity consisting of frequent LP, VT, and TR events continued to be detected during December, occasionally accompanied by gas-and-steam emissions that rose 200-2,000 above the crater. Between 0758 and 0816 on 8 December ash plumes rose approximately 1 km above the summit and drifted WNW; there were no reports of ashfall. During 9-10 December gas-and-ash emissions rose 1.1 km above the crater and drifted NW, causing minor ashfall in Conocoto-Pichincha. A gas-and-ash plume during 10-11 December rose 2 km above the crater and drifted W and NW; there were reports of ashfall at the S area of the Cotopaxi National Park. During 14-15 December gas-and-ash plumes rose 500 m above the crater and drifted W, causing moderate ashfall in the Cotopaxi National Park. During 19-20 December the Washington VAAC reported ash emissions to 1.1 km above the crater that drifted N and NW (figure 22), which caused ashfall in San Pedro de Taboada (40 km N), Uyumbicho (30 km NNW), Guitig Alta, Guitig Baja, Conocoto, Sur de Quito, Chimbacalle (48 km NNW), La Magdalena (48 km NNW), Barrio Nuevo, Villaflora, Miraflores, and La Floresta (50 km N). IG also reported an ash plume to 1.5 km above the crater that drifted 60 km NNW, resulting in ashfall in the Mejía, Rumiñahui, and Quito areas on 20 December. During 21-22 December gas-and-ash emissions rose 500 m above the crater and drifted SE, E, and W; ashfall was reported at Caspi, the S entrance to the national park.

Figure 22. Webcam image showing a gas-and-ash plume rising above Cotopaxi on 20 December 2022. Photo has been color corrected. Courtesy of IG (INFORME DIARIO DEL VOLCAN COTOPAXI No. 2022-060).

Near daily gas-and-steam emissions continued to be reported during January 2023, rising 500-1,000 m above the crater. Seismicity continued with frequent LP and TR events and occasional VT events. The Washington VAAC reported ash emissions that rose 200-2,000 m above the crater and drifted in multiple directions. During 5-6 January a GOES 16 satellite image acquired at 0500 showed a diffuse ash cloud 1 km above the crater that drifted NW; minor ashfall was observed over national park. During 9-13 January the Washington VAAC reported ash emissions rising as high as 2 km above the crater and drifting NW, SW, and W; reports of ashfall came from Colcas, San Ramón, San Agustín de Callo, Mulaló, Mulaló Grande, San Antonio, San Ramón (127 km W), Ticatilin (15 km SW), and MAE Norte, 18 km N of the volcano. Additionally, a sulfur odor was reported during 11-12 December in the towns of Ticatilín and Control Caspi (20 km WSW). During 16-19, 25-27, and 29-30 January light ashfall was reported in Mulaló, San Juan de Pastocalle (20 km WSW), Aláquez, San Agustín de Callo (18 km WSW), Villacís, Ticatilín, Caspi, San Ramon, Chillos (33 km SW), Langualó, San Isidro Alto (20 km SW), and on the N side of the national park. A significant increase in size and density of ash emissions was visible in satellite images at 0820 on 30 January; the plumes rose as high as 2.5 km above the crater rim and drifted SW, S, and SE (figure 23).

Figure 23. Webcam image showing a strong ash plume that rose above Cotopaxi on 30 January 2023 and drifted SE. Courtesy of IG (INFORME DIARIO DEL VOLCAN COTOPAXI No. 2023-030).

Additional satellite data. Scientists measured sulfur dioxide emissions using a mobile Differential Optical Absorption Spectroscopy (DOAS) instrument on 22 October; emissions were 1,580 tons/day (t/d). Using Sentinel-5P TROPOMI data process by the MOUNTS system, and the DOAS network, IG noted increased degassing beginning on 5 November with sulfur dioxide emissions ranging from 94 tons to 3,493 tons, the latter of which was measured on 26 November. During December levels of sulfur dioxide fluctuated between 119 and 5,753 tons, the latter of which was measured on 25 December. Similar variation was seen in January, with SO₂ values measured between 2 tons and 4,718 tons, with the maximum on 8 January. Sentinel-2 infrared satellite images showed a small thermal anomaly at the summit crater on four clear days on 23 October and 2, 17, and 22 December (figure 24).

Figure 24. Sentinel-2 infrared satellite imagery showing a small thermal anomaly at the summit crater of Cotopaxi on 23 October 2022 (top left), 2 December 2022 (top right), 17 December 2022 (bottom left), and 22 December 2022 (bottom right). Images using “Atmospheric penetration” (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN), Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Servicio Nacional de Gestion de Riesgos y Emergencias (SNGRE), Samborondón, Ecuador (URL: https://www.gestionderiesgos.gob.ec/); 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); MOUNTS Project (Monitoring Unrest From Space), an operational monitoring system for volcanoes using Sentinel satellite data from ESA's Copernicus, Open Access Hub, hosted at UNAM and CT TU-Berlin (URL: http://www.mounts-project.com/home); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

August 2023 (BGVN 48:08) Cite this Report

Ash emissions, ashfall, and occasional lahars during February-July 2023

Cotopaxi is a steep-sided cone capped by nested summit craters, located in Ecuador. The largest cone is about 550 x 800 m. Explosive eruptions have been accompanied by pyroclastic flows and lahars, the latter of which frequently affect adjacent valleys. The most recent eruption began in October 2022 and consisted of ash plumes and ashfall (BGVN 48:02). This report covers activity during February through July 2023, which was characterized by similar activity. Information comes from Ecuador's Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN) and the Washington Volcanic Ash Advisory Center (VAAC).

Activity during February consisted of constant gas-and-steam emissions that rose 200-2,500 m above the crater and drifted in multiple directions, and seismicity that was characterized by daily tremor events associated with emissions (TR) and long-period events (LP), and less frequent volcano-tectonic events (VT) and very long-period events (VLP). The Washington VAAC reported during 31 January to 4 February that gas-and-ash emissions rose 1.1-2.5 km above the summit and drifted in multiple directions. Beginning at 0100 on 2 February IG reported an increase in seismic signals associated with ash emissions that rose 1.3 km above the summit and drifted NW (figure 25). There were reports of ashfall in the N part of the Cotopaxi National Park in Quito and Mejía, particularly in Amaguaña (35 km NNW), La Armenia, Quitumbe (41 km NNW), Conocoto (41 km N), Guamaní (42 km NNW), La Ecuatoriana (44 km NNW), Turubamba (43 km NNW), Chillogallo (47 km NNW), La Magdalena (48 km NNW), Machachi, Tambillo (32 km NNW), Alóag (28 km NW), Cutuglahua (35 km NNW), Uyumbicho (30 km NNW), Aloasí (24 km NW), and El Chaupi (24 km WNW) during 2-4 February. On the morning of 3 February continuous gas-and-ash emissions were reported, resulting in slight ashfall in Tambillo, Chillogallo, and La Armenia 2 and later that same day in Guamaní, Turubamba, Chillogallo, La Ecuatoriana, Quitumbe, Tambillo, Machachi, Aloasí, Aloag (28 km NW), and Conocoto.

Figure 25. Webcam image of a gray ash plume rising above Cotopaxi on 2 February 2023. Courtesy of IG-EPN (IG Al Instante Informativo VOLCÁN COTOPAXI No 2023-023).

During 9-10, 12-19, and 26-27 February the Washington VAAC reported that gas-and-ash emissions rose 200-1,400 m above the crater and drifted W, NW, S, and SE. At 1800 on 9 February an ash plume rose 2 km above the summit and drifted W according to IG-EPN. Slight ashfall was reported in Tambillo. During the night and early morning of 11-12 February gas-and-ash emissions rose less than 500 m above the crater and drifted SW; there were some reports of light ashfall in El Chasqui (17 km W), Mulaló (19 km SW) and San Juan de Pastocalle (20 km WSW). Around 0810 the BREF seismic station recorded an increase in tremor energy, which is associated with ash emissions. During 13-14 February several gas-and-ash emissions rose as high as 1 km and drifted W and SW, resulting in minor ashfall in Mulaló, San Agustín (11 km W), Ticatilín (15 km WSW), San Ramón (17 km SW), Control Caspi (20 km WSW), and Pastocalle (22 km W). Starting around 0630 on 15 February ashfall was observed in Mulaló 2, San Ramón (17 km SW), Ticatilín (15 km SW), and San Agustín del Callo (18 km WSW). According to the Network of Volcanic Observers (ROVE), ashfall was reported in Latacunga canton (18 km WSW) during the night of 15 February. On 16 February at 0600 ash emissions rose 1 km above the summit and drifted E, based on surveillance cameras (figure 26). Starting at 0230 on 18 February IG reported an increase in seismic signals that were associated with ash emissions; by 0650 satellite images showed an ash plume that rose 800 m above the summit and drifted SE. Around 0030 on 19 February an ash plume rose 1.1 km above the crater and drifted S. During the morning of 27 February gas-and-ash emissions rose 1 km above the summit and drifted SE; light ashfall was reported in Pichincha en Rumiñahui (61 km N), Rumipamba Vallecito (55 km N), Conocoto, Pedregal (60 km N), Guamaní, Quitumbe, La Ecuatoriana, Chillogallo, Urubamba (Santo Tomas, 40 km NNW), La Magdalena (Barrio Nuevo, Villaflora), and San Bartolo. On 28 February at 1430 surveillance cameras showed an ash plume that rose 500 m above the summit and drifted SW.

Figure 26. Webcam image of an ash emission rising 1 km above the summit of Cotopaxi and drifting E on 16 February 2023. Photo has been color corrected. Courtesy of IG-EPN (IG Al Instante Informativo VOLCÁN COTOPAXI No 2023-031).

Ash emissions continued during March, rising 1-1.5 km above the summit, and drifting in different directions. Seismicity consisted of daily LP and near-daily TR events and less frequent VT and VLP events. Intermittent gas-and-steam emissions rose 100-1,500 m and drifted in different directions. According to the Washington VAAC ash emissions rose 500-2,000 m above the summit and drifted in multiple directions during 28 February to 3 March, 4-5, 18-19, and 24-29 March. Light ashfall was reported in Mulaló during 28 February to 1 March and 5-6 March. On 19 March at 0900 surveillance cameras captured an ash emission that rose 1 km above the summit and drifted E (figure 27). A thin ash cloud was observed in GOES-16 satellite images rising 500 m above the summit and drifted SW on 25 March. Around 0310 on 28 March a GOES-16 satellite image showed an ash cloud rising 1 km above the summit and drifted N. As a result, light ashfall was observed in Machachi and on the N flank of the volcano and in El Chasqui, Latacunga, and the S flank of the volcano during 28-29 March.

Figure 27. Webcam image of an ash emission rising 1 km above the summit of Cotopaxi and drifting E on 19 March 2023. Photo has been color corrected. Courtesy of IG-EPN (IG Al Instante Informativo VOLCÁN COTOPAXI No 2023-036).

Gas-and-steam emissions continued during April, rising 200-1,500 m above the crater, and drifting in different directions. Seismicity consisted of daily LP and near-daily TR events and less frequent VT events. Gas-and-ash emissions rose 200-1,500 m above the crater and drifted W, SW, E, and SE. During 4-6 and 9-10 April the Washington VAAC reported that ash emissions rose 200-1,100 m above the crater and drifted SW, SE, and E. A lahar occurred on the upper W flank on 5 April due to moderate rainfall. Light ashfall was reported in Mulaló and San Agustín during 5-6 April. On 8 April at 0900 a surveillance camera showed an ash plume that rose 500 m above the summit and drifted E and on 10 April at 0500 an ash plume rose 1.1 km above the summit and drifted W. During the night of 11 April, a small, secondary lahar was descended the Agualongo drainage. Another lahar was recorded during 12-13 April, though it was not observed due to cloud cover. On 15 April a small lahar was observed on the NW flank. The BNAS seismic station recorded an increase in seismic signal at 1600 on 22 April that corresponded to a small lahar, which descended the Cutzulao/Agualongo drainages. During 23-24 April the Washington VAAC reported ash emissions that rose as high as 2,028 m above the summit and drifted NE. According to IG-EPN surveillance camera showed an ash plume rising 3 km above the summit and drifted NE at 0953 (figure 28). During 24-25 and 28-30 April ash emissions rose 200-2,000 m above the crater and drifted NE and W. According to ROVE, ashfall was reported in the S area of the national park around 1805.

Figure 28. Webcam image of an ash emission rising 3 km above the summit of Cotopaxi and drifting NE on 24 April 2023. Photo has been color corrected. Courtesy of IG-EPN (INFORME DIARIO DEL VOLCAN COTOPAXI No. 2023-114).

Similar activity continued during May. Seismicity was characterized by daily LP and near-daily TR events and occasional VT events. Gas-and-steam emissions rose 200-1,500 m above the summit and drifted in different directions. Gas-and-ash emissions rose 200-1,400 m above the crater and drifted W, SW, NW, and SE. On 1 May at 0130 and on 8 May the BNAS seismic station recorded high-frequency seismic signals that corresponded to small lahars that remained within the Cotopaxi National Park. An ash plume was observed in a GOES-16 satellite image around 1100 that rose 1.5 km above the summit and drifted WNW, reaching Manabí on 4 May. Light ashfall was reported in Mulaló during 6-7 May. At 0600 on 7 May an ash plume rose 500-1,100 m above the summit and drifted W and SW. Ashfall was reported in Mulaló. The Washington VAAC reported that ash emissions rose 800 m above the summit and drifted NW during 7-8 May. During 9-10 May a moderate ash plume rose 2-3 km above the summit and drifted SW, N, and NE. Ashfall was reported in San Joaquín (SW) and San Agustín de Callo (SW). Another ash plume rose 2 km above the summit and drifted SE at 0749 on 12 May (figure 29). Intermittent ash plumes rose 1-3 km above the summit starting at 0510 on 18 May and drifted NE. As a result, light ashfall was reported in Machachi. Continuous ash emissions were recorded during the morning of 26 May, rising 1.5-2 km above the crater and drifting W and NW. Ash emissions rose as high as 1.1 km above the summit and drifted W and SW during 29-30 May according to the Washington VAAC. At 0600 surveillance cameras and satellite images showed an ash plume rising 1.1 km above the crater and drifted as far as 40 km W. Ashfall was reported in Pastocalle. During 30-31 May an ash emission rose 500 m above the summit and drifted W, based on a Washington VAAC report.

Figure 29. Webcam image showing an ash plume rising as high as 2 km above the summit of Cotopaxi on 12 May 2023. Courtesy of IG-EPN (IG Al Instante Informativo VOLCÁN COTOPAXI No 2023-048).

During June, gas-and-steam emissions rose 100-500 m above the summit and seismicity continued with daily LP and near-daily TR events and less frequent VT events. Gas-and-ash emissions rose 100-1,000 m above the summit and drifted in several directions. Light ashfall was reported in San Agustín de Callo and San Ramón during 1-2 June. Ash emissions rose 1 km above the summit and drifted SW during 1746-2000 on 3 June. According to the Washington VAAC, ash emissions rose 800 m above the summit and drifted SW during 4-5 June. A seismic signal was recorded on the NW flank on 8 June that IG reported was possibly associated with a small lahar. Around 0900 on 21 June an ash cloud rose less than 500 m above the summit and drifted over the W and SW flanks due to strong winds; light ashfall was reported in the Cotopaxi National Park. Starting around 1400 on 27 June through 28 June a high-frequency tremor signal was recorded at the BREF and BNAS seismic stations associated with small lahar signals moving through the Agualongo Creek; the flows were limited to the Cotopaxi National Park. Around 1300 on 29 June two seismic signals relating to small lahars were recorded on the W flank of the volcano moving down the Agualongo Creek.

Activity during July was relatively low and was mainly characterized by gas-and-steam emissions rising 100-1,800 m above the summit and seismicity consisting of daily LP and near-daily TR events and less frequent VT events. During the afternoon of 1 July, a high-frequency seismic signal was recorded on the NW flank relating to a small lahar. Gas-and-ash emissions rose 100-1,100 m above the summit and drifted SW and W. The Washington VAAC reported an ash emission that rose 500 m above the summit and drifted SW during 2-3 July. Light ashfall was reported in Mulaló, including Rosal, Ticatilín, San Agustín del Callo, San Ramón, and Rumipamba de Villacis. On 5 July around 0700 an ash plume rose 600 m above the summit and drifted SW, based on a GOES-16 satellite images. According to IG-EPN cameras and ROVE reports, remobilized ash was observed on the W flank resulting from strong winds during 18-19 July.

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN), Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); 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).