Most Recent Weekly Report: 2 August-8 August 2023 Cite this Report

AVO reported that the last time possible explosions were detected at Semisopochnoi’s Mount Young in monitoring data was 25 April and the last ash emission occurred on 5 May. Since then, earthquake activity has been low and steaming from the N crater continued. On 3 August the Volcano Alert Level was lowered to Normal (the lowest level on a four-level scale) and the Aviation Color Code was lowered to Green (the lowest color on a four-color scale).

Source: US Geological Survey Alaska Volcano Observatory (AVO)

Most Recent Bulletin Report: June 2023 (BGVN 48:06) Cite this Report

Weekly Reports - Index

2023: January | February | March | April | May | August
2022: January | February | March | April | May | June | July | August | September | November | December
2021: February | March | April | May | June | July | August | September | October | November | December
2020: January | February | March | April | November
2019: July | August | September | December
2018: September | October | November
2015: April | May
2014: June

2 August-8 August 2023 Cite this Report

AVO reported that the last time possible explosions were detected at Semisopochnoi’s Mount Young in monitoring data was 25 April and the last ash emission occurred on 5 May. Since then, earthquake activity has been low and steaming from the N crater continued. On 3 August the Volcano Alert Level was lowered to Normal (the lowest level on a four-level scale) and the Aviation Color Code was lowered to Green (the lowest color on a four-color scale).

Source: US Geological Survey Alaska Volcano Observatory (AVO)

31 May-6 June 2023 Cite this Report

AVO reported that low-level unrest continued at Semisopochnoi during 30 May-6 June. Seismicity remained low and few earthquakes were detected. Minor steam emissions were occasionally visible in webcam images on 31 May and 2 and 4 June. The Volcano Alert Level remained at Advisory (the second highest on a four-level scale) and the Aviation Color Code remained at Yellow (the second highest color on a four-color scale).

Source: US Geological Survey Alaska Volcano Observatory (AVO)

17 May-23 May 2023 Cite this Report

On 17 May AVO reported that eruptive activity at Semisopochnoi had declined during the previous week, though seismicity remained slightly elevated and low-level steaming continued from Mount Young. Ash emissions had last occurred on 5 May leaving minor deposits on the NW flank of Mount Young’s N crater. The Aviation Color Code was lowered to Yellow (the second lowest level on a four-color scale) and the Volcano Alert Level was lowered to Advisory (the second highest level on a four-level scale). Intense gas emissions were periodically visible in webcam images during 17-18 and 20-21 May. On 22 May a weak sulfur dioxide signal was detected, suggestive of low-level degassing.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

10 May-16 May 2023 Cite this Report

AVO reported that low-level unrest continued at Semisopochnoi during 9-16 May. Weather clouds mostly obscured satellite and webcam views. Volcanic tremor was detected during 9-10 May; there was no evidence of explosive or earthquake activity in the geophysics data during the rest of the week. The Volcano Alert Level remained at Watch (the second highest level on a four-level scale) and the Aviation Color Code remained at Orange (the second highest level on a four-color scale).

Source: US Geological Survey Alaska Volcano Observatory (AVO)

3 May-9 May 2023 Cite this Report

AVO reported that low-level unrest continued at Semisopochnoi during 2-9 May, though it was seismically quiet for the most part, with only occasional small earthquakes. Cloudy weather often obscured satellite views. Steam emissions from the N crater of Mount Young were visible in partially clear webcam images during 3-6 May. A new NW-flank ash deposit was present on fresh snow; no signs of explosive activity were seen in geophysical data. The Volcano Alert Level remained at Watch (the second highest level on a four-level scale) and the Aviation Color Code remained at Orange (the second highest level on a four-color scale).

Source: US Geological Survey Alaska Volcano Observatory (AVO)

26 April-2 May 2023 Cite this Report

AVO reported that low-level unrest continued at Semisopochnoi during 26 April-2 May. Seismicity was at low levels, and a few small local earthquakes were recorded during 28-29 April. Daily minor steam emissions were seen rising from Mount Young, though cloudy weather sometimes prevented webcam and satellite views. Ash deposits near the crater rim were visible during 27-28 April, though it was unclear if they were recent. The Volcano Alert Level remained at Watch (the second highest level on a four-level scale) and the Aviation Color Code remained at Orange (the second highest color on a four-color scale).

Source: US Geological Survey Alaska Volcano Observatory (AVO)

19 April-25 April 2023 Cite this Report

AVO reported that low-level unrest continued at Semisopochnoi during 18-25 April. Daily periods or bursts of tremor and occasional low-frequency earthquakes were detected during the week. Small explosions were detected in seismic and infrasound data during 18-19 and 24-25 April. Cloudy weather prevented webcam and satellite views on most days. Possible recent ash deposits on Mount Young’s crater rim were visible in clear webcam images during 22-23 April, and steam emissions from the active N crater were visible during 22-25 April. The Volcano Alert Level remained at Watch (the second highest level on a four-level scale) and the Aviation Color Code remained at Orange (the second highest color on a four-color scale).

Source: US Geological Survey Alaska Volcano Observatory (AVO)

12 April-18 April 2023 Cite this Report

AVO reported that low-level unrest continued at Semisopochnoi during 12-18 April. No eruptive activity was observed, though some weak seismicity was recorded. Cloudy weather often prevented webcam and satellite views during most of the week. The Volcano Alert Level remained at Watch (the second highest level on a four-level scale) and the Aviation Color Code remained at Orange (the second highest color on a four-color scale).

Source: US Geological Survey Alaska Volcano Observatory (AVO)

5 April-11 April 2023 Cite this Report

AVO reported that low-level unrest continued at Semisopochnoi during 5-11 April. An extensive low-altitude steam plume drifted more than 400 km during 4-6 April and was likely related to volcanic emissions but did not appear to contain ash. Several small explosions were detected on 5 April, and periods of seismic tremor and local earthquakes were recorded during 5-6 April, but seismicity was quiet during the rest of the week. Clouds obscured webcam and satellite views during 7-10 April. The Volcano Alert Level remained at Watch (the second highest level on a four-level scale) and the Aviation Color Code remained at Orange (the second highest color on a four-color scale).

Source: US Geological Survey Alaska Volcano Observatory (AVO)

29 March-4 April 2023 Cite this Report

AVO reported that low-level unrest continued at Semisopochnoi during 28 March-4 April. Steam emissions from the N crater of Mount Young were visible in webcam images during 29-30 March; weather clouds obscured webcam and satellite views during most of the week. Minor seismic and infrasound signals were recorded during 31 March-1 April that may or may not have been related to activity at N crater. Local earthquakes and periods of tremor occurred during 1-2 April. A 200-km-long gas-and-steam emission was visible at low altitudes during the night of 3 April; no ash signatures were detected in the cloud. The Volcano Alert Level remained at Watch (the second highest level on a four-level scale) and the Aviation Color Code remained at Orange (the second highest color on a four-color scale).

Source: US Geological Survey Alaska Volcano Observatory (AVO)

22 March-28 March 2023 Cite this Report

AVO reported that low-level unrest continued at Semisopochnoi during 22-28 March. Steam emissions from the N crater of Mount Young were visible during 22 and 26-27 March. No explosive activity was detected in seismic or infrasound data. The Volcano Alert Level remained at Watch (the second highest level on a four-level scale) and the Aviation Color Code remained at Orange (the second highest color on a four-color scale).

Source: US Geological Survey Alaska Volcano Observatory (AVO)

15 March-21 March 2023 Cite this Report

AVO reported that low-level ash emissions from the N crater of Semisopochnoi’s Mount Young were observed in several web camera images during 18-19 March. Small explosions and volcanic tremor had also resumed. Ash emissions were not detected in satellite images, though on 18 March a robust steam-and-gas plume was visible drifting 150 km from the N crater. AVO raised the Volcano Alert Level to Watch (the second highest level on a four-level scale) and the Aviation Color Code to Orange (the second highest color on a four-color scale) on 19 March. Low-level seismicity continued during 20-21 March and one small explosion was detected in seismic and infrasound data. Clouds obscured webcam and satellite views.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

15 February-21 February 2023 Cite this Report

AVO reported that seismicity at Semisopochnoi’s Mount Young was low during 14-21 February, and steam emissions were visible in webcam images almost daily. On 22 February the Aviation Color Code was lowered to Yellow (the second lowest level on a four-color scale) and the Volcano Alert Level was lowered to Advisory (the second highest level on a four-level scale). AVO noted that no significant tremor, ash emissions, or explosive activity had been recorded since late January.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

8 February-14 February 2023 Cite this Report

AVO reported that eruptive activity at Semisopochnoi’s Mount Young continued during 7-14 February. Seismicity was low, and a few local earthquakes were recorded during 7-8 February. Steam emissions were visible in webcam images almost daily though views were often cloudy. The Aviation Color Code remained at Orange (the second highest level on a four-color scale) and the Volcano Alert Level remained at Watch (the second highest level on a four-level scale).

Source: US Geological Survey Alaska Volcano Observatory (AVO)

1 February-7 February 2023 Cite this Report

AVO reported that eruptive activity at Semisopochnoi’s Mount Young continued during 1-7 February. Seismicity was elevated and daily weak tremor was recorded. Minor steam emissions were visible in webcam images during 3-7 February. The Aviation Color Code remained at Orange (the second highest level on a four-color scale) and the Volcano Alert Level remained at Watch (the second highest level on a four-level scale).

Source: US Geological Survey Alaska Volcano Observatory (AVO)

25 January-31 January 2023 Cite this Report

AVO reported that eruptive activity at Semisopochnoi’s Mount Young was ongoing during 24-31 January. Seismicity was elevated and daily weak tremor was recorded. Minor steam emissions were visible in webcam images on 24 January and 29-30 January. Discolored snow observed at the summit in webcam images during 28-29 January possibly indicated minor, low-level explosive activity at the vent. The Aviation Color Code remained at Orange (the second highest level on a four-color scale) and the Volcano Alert Level remained at Watch (the second highest level on a four-level scale).

Source: US Geological Survey Alaska Volcano Observatory (AVO)

18 January-24 January 2023 Cite this Report

AVO reported that eruptive activity at Semisopochnoi’s Mount Young was ongoing during 17-24 January. Seismicity was elevated with daily small local earthquakes and occasional weak tremor. Minor steam emissions were visible in webcam images during 17-19 January; no activity was identified in cloudy satellite views during the rest of the week. The Aviation Color Code remained at Orange (the second highest level on a four-color scale) and the Volcano Alert Level remained at Watch (the second highest level on a four-level scale).

Source: US Geological Survey Alaska Volcano Observatory (AVO)

11 January-17 January 2023 Cite this Report

AVO reported that eruptive activity at Semisopochnoi’s Mount Young was ongoing during 10-17 January. Seismicity was elevated with small local earthquakes and weak tremor. Minor steam emissions were visible in webcam images during 11 and 16-17 January. Several small explosion signals were detected by local seismic and infrasound sensors during 16-17 January; no activity was identified in cloudy satellite views. The Aviation Color Code remained at Orange (the second highest level on a four-color scale) and the Volcano Alert Level remained at Watch (the second highest level on a four-level scale).

Source: US Geological Survey Alaska Volcano Observatory (AVO)

4 January-10 January 2023 Cite this Report

AVO reported that eruptive activity at Semisopochnoi’s Mount Young, formerly Mount Cerberus, was ongoing during 4-10 January. Daily minor steam emissions were visible in webcam views. Seismicity was above background levels; low-level explosive activity was detected in geophysical data during 4-5 January with elevated seismicity and infrasound signals observed on local stations. Volcanic tremor was detected during 7-9 January, and very weak explosive activity was detected in seismic and infrasound data on 9 January. The Aviation Color Code remained at Orange (the second highest level on a four-color scale) and the Volcano Alert Level remained at Watch (the second highest level on a four-level scale).

Source: US Geological Survey Alaska Volcano Observatory (AVO)

28 December-3 January 2023 Cite this Report

AVO reported that fresh ash deposits on the flanks of Semisopochnoi’s Mount Cerberus were visible in webcam images during 27-28 December, indicating that activity at the N crater had resumed on 27 December. The deposits extended about 1 km from the vent. Ash plumes were not visible in satellite and webcam images, though a persistent steam plume rising as high as 1.5 km (5,000 ft) a.s.l. may have been carrying minor amounts of ash. Increased seismicity, including seismic tremor, had been recorded during the previous week. AVO raised the Aviation Color Code to Orange (the second highest level on a four-color scale) and the Volcano Alert Level to Watch (the second highest level on a four-level scale).

Seismic tremor and explosions were recorded during 30-31 December. Satellite and webcam images were obscured by clouds; no plumes were observed above the meteorological cloud deck around 4.7 km (15,000 ft) a.s.l., so any ash emissions would have been at low altitudes. Possible explosions were detected during 31 December 2022-1 January 2023. A small ash deposit extending around 2 km SSW of the N crater were visible in satellite and webcam images. A likely explosion occurred during 1-2 January based on elevated seismicity recorded on local seismometers and an infrasound signal recorded minutes later by an array at Adak. Minor steam-and-gas emissions were visible in partly cloudy satellite images. Low-level explosive activity persisted during 2-3 January, with minor steam emissions and a new ash deposit visible in webcam images.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

23 November-29 November 2022 Cite this Report

On 23 November AVO lowered the Aviation Color Code for Semisopochnoi to Yellow (the second lowest level on a four-color scale) and the Volcano Alert Level to Advisory (the second lowest level on a four-level scale) noting that no ash emissions or explosive activity had been detected since 7 November. Seismicity had decreased, though remained at elevated levels. Steam emissions from the active vent in the N crater of Mount Cerberus persisted.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

16 November-22 November 2022 Cite this Report

AVO reported ongoing low-level seismicity at Semisopochnoi characterized by intermittent seismic tremor and occasional low-frequency earthquakes during 16-22 November. Satellite and webcam views were mostly obscured by weather clouds, though during clear views continuous gas-and-steam emissions from the N crater of Mount Cerberus were visible. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

2 November-8 November 2022 Cite this Report

On 7 November AVO stated that small explosions at the N crater of Mount Cerberus at Semisopochnoi had been detected in geophysical data during the previous week and volcanic tremor simultaneously resumed. Though ash emissions were not visually observed, the type of unrest was similar to previous periods associated with ash emissions below 3 km (10,000 ft) a.s.l.; the Aviation Color Code was raised to Orange (the second highest level on a four-color scale) and the Volcano Alert Level was raised to Watch (the second highest level on a four-level scale). Seismicity was low during 7-8 November, and clouds prevented webcam and satellite images.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

28 September-4 October 2022 Cite this Report

On 29 September AVO lowered the Aviation Color Code for Semisopochnoi to Yellow (the second lowest level on a four-color scale) and the Volcano Alert Level to Advisory (the second lowest level on a four-level scale) noting that no ash emissions or explosive activity had been detected since 14 September. Seismicity had decreased, though remained at elevated levels. Steam emissions from the active vent in the N crater of Mount Cerberus persisted. Seismic tremor and a small explosion were detected in seismic and infrasound data during 3-4 October.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

21 September-27 September 2022 Cite this Report

AVO reported that the eruption at Semisopochnoi was ongoing during 13-20 September. Seismicity remained elevated with intermittent periods of low-amplitude tremor. Steam emissions from the active vent in the N crater of Mount Cerebus were visible in webcam views on most days. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

14 September-20 September 2022 Cite this Report

AVO reported that the eruption at Semisopochnoi was ongoing during 13-20 September. Seismicity remained elevated and characterized by intermittent tremor. Low-level ash emissions from the N crater of Mount Cerberus were occasionally visible in mostly cloudy webcam views during 13-15 September. Possible fresh local ashfall was seen in webcam images during 16-17 September. Steam emissions were visible in webcam views during 19-20 September. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

7 September-13 September 2022 Cite this Report

AVO reported ongoing low-level seismicity and occasional steam emissions at Semisopochnoi during 6-13 September. Satellite and webcam views were mostly obscured by weather clouds. Explosions were recorded during 12-13 September and ash emissions drifting 8 km SE at an altitude of 1.2 km (4,000 ft) a.s.l. were visible in satellite and webcam images. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

31 August-6 September 2022 Cite this Report

AVO reported ongoing low-level seismicity and steam emissions at Semisopochnoi during 30 August-6 September. Weakly elevated surface temperatures were identified in satellite images during 1-2 September. Several small ash eruptions from the N crater of Mount Cerberus were visible in webcam images during 2-3 September and detected in seismic and infrasound data. Satellite and webcam views were cloudy during 4-5 September. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

24 August-30 August 2022 Cite this Report

AVO reported ongoing low-level seismicity and steam emissions at Semisopochnoi during 23-30 August. Satellite images were mostly cloudy; steam emissions were seen almost daily in webcam images. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

17 August-23 August 2022 Cite this Report

AVO reported that at 1347 on 21 August a short-lived explosion at Semisopochnoi's North Cerberus cone produced an ash emission that was visible in webcam images. The ash emission was not visible in satellite images due to weather clouds, indicating that they did not rise above 6 km (20,000 ft) a.s.l. This was the first explosion detected since 12 June. The Aviation Color Code was raised to Orange and the Volcano Alert Level was raised to Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

6 July-12 July 2022 Cite this Report

AVO reported ongoing low-level seismicity, steam emissions, and sulfur dioxide emissions at Semisopochnoi, though no explosive activity had been detected since 12 June. The Aviation Color Code was lowered to Yellow and the Volcano Alert Level was lowered to Advisory on 7 July.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

29 June-5 July 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone likely continued during 29 June-5 July. Seismicity was quiet. Weather clouds often prevented satellite and webcam views; sulfur dioxide emissions were detected in satellite images on 28 and 30 June. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

22 June-28 June 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 21-27 June. Periods of low-amplitude tremor and a few small low-frequency earthquakes were recorded by the seismic network. Weather clouds often prevented satellite and webcam views; sulfur dioxide emissions were detected in satellite images during 23-24 June and a robust steam plume was visible in webcam images during 25-26 June. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

15 June-21 June 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 14-21 June. Periods of weak tremor were recorded by the seismic network. Weather clouds often prevented satellite and webcam views; minor steam emissions were visible in one clear webcam image on 17 June. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

8 June-14 June 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 7-14 June. Seismicity remained elevated with intermittent tremor and occasional explosions. Minor ash emissions rising to low altitudes (less than 1.5 km (5,000 ft) a.s.l.) were visible in webcam images on most days; weather clouds sometimes prevented satellite and webcam views. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

1 June-7 June 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 31 May-6 June. Seismicity remained elevated with intermittent tremor; one explosion was recorded during 31 May-1 June and multiple small explosions were recorded during 4-6 June. Weather clouds mostly prevented satellite and webcam views. Minor ash emissions rising to low heights were visible in webcam images during 6-7 June. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

25 May-31 May 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 24-31 May. Seismicity continued to be elevated with intermittent tremor and several daily explosions recorded by infrasound and seismic instruments. Weather clouds often prevented satellite and webcam views; sporadic ash emissions were visible during 27-28 May and likely occurred on other days as well. Sulfur dioxide emissions were detected during 27-29 May, and elevated surface temperatures were identified during 28-29 May. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

18 May-24 May 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 17-24 May. Seismicity continued to be elevated with intermittent tremor and several daily explosions recorded by infrasound and seismic instruments. Low-level ash emissions were visible in clear satellite images and webcam views; clouds sometimes obscured views towards the end of the week. A sulfur dioxide plume drifted 240 km N during 17-18 May, and sulfur dioxide emissions were detected during 23-24 May. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

11 May-17 May 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 10-17 May. Seismicity continued to be elevated with intermittent tremor detected by the seismic network. Several daily explosions were recorded in infrasound and seismic data. Daily low-level ash emissions were visible in clear satellite images and webcam views. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

4 May-10 May 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 3-10 May. Seismicity continued to be elevated with intermittent tremor detected by the seismic network. Several daily explosions were recorded during 8-10 May. Weather clouds obscured views of the volcano, though daily minor, low-level ash emissions were seen in webcam views. A pilot reported a low-level eruption cloud on 9 May. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

27 April-3 May 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 26 April-3 May. Tremor continued to be detected by the seismic network. Weather clouds obscured views of the volcano on most days, though occasional views during 27-29 May revealed new ash deposits on the flanks. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

20 April-26 April 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 19-26 April. Periods of seismic tremor were occasionally detected and small explosions were recorded in seismic data during 19-20 April. Minor, low-level, plumes with low ash content were visible in webcam images through each day during 19-20 April, with occasional more energetic ash plumes. Weather cloud cover often hindered webcam and satellite views during the rest of the week. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

13 April-19 April 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 12-19 April. Periods of seismic tremor were detected daily and occasional small explosions were recorded in seismic and regional infrasound data on most days. Weather cloud cover often hindered webcam and satellite views; minor, low-level ash plumes were visible in webcam images during 16-19 April. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

6 April-12 April 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 6-12 April. Periods of seismic tremor were detected daily and occasional small explosions were recorded on most days in seismic and regional infrasound data. Partly-to-mostly-cloudy webcam views and occasional satellite images showed daily short-lived ash bursts and more continuous steam emissions. Steam plumes drifted as far as 70 km SW during 6-7 April. A low-level steam plume drifted more than 100 km at altitudes less than 1.5 km (5,000 ft) a.s.l. during 7-8 April. A low-level ash plume drifted up to 80 km WNW on 8 April. Local ash deposits were occasionally visible. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

30 March-5 April 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 30 March-4 April. Periods of seismic tremor and occasional small explosions were detected daily in seismic and regional infrasound data. Daily minor ash emissions rose no higher than 3 km (10,000 ft) a.s.l. and occasional steam emissions were visible in webcam and satellite images; clouds sometimes prevented satellite views. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

23 March-29 March 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 22-29 March. Periods of seismic tremor and occasional small explosions were detected daily in seismic and regional infrasound data. Daily minor ash emissions and occasional steam emissions were visible in webcam and satellite images; clouds sometimes prevented satellite views. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

16 March-22 March 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 15-22 March. Periods of seismic tremor and occasional small explosions were detected daily in seismic and regional infrasound data. Daily minor ash emissions and occasional steam emissions were visible in webcam images. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

9 March-15 March 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 8-15 March. Seismic tremor and sometimes numerous daily explosions were detected in seismic and regional infrasound data. Minor ash emissions were visible in webcam images during 8-9 and 14-15 March; likely plumes on other days may have been hidden due to clouds. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

2 March-8 March 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 1-8 March. Local seismic and infrasound instruments recorded daily small explosions. Weather clouds often prevented satellite and webcam views of the volcano, though explosions likely produced low ash clouds from the summit and they were visually confirmed during 4-8 March. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

23 February-1 March 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 23 February-1 March. Seismicity was elevated with low-level tremor and daily small explosions. Weather clouds sometimes prevented satellite and webcam views of the volcano, though low ash clouds from the summit were visible on most days. Ash deposits on the N flank were visible in a few webcam images. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

16 February-22 February 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 16-22 February. Seismicity was elevated with low-level tremor, and occasional explosions, detected almost daily in both seismic and infrasound data. Weather clouds prevented satellite and webcam views of the volcano. Low-level ash clouds from the summit were observed in webcam images drifting N during 21-22 February. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

9 February-15 February 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 9-15 February. Seismicity was elevated with low-level tremor, and several small explosions were detected daily in both seismic and infrasound data, through 13 February. Steam and low-level ash emissions likely occurred daily, though due to weather clouds they were not confirmed in satellite and webcam images during 13-14 February. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

2 February-8 February 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 1-8 February. Seismicity was elevated, and explosions were occasionally detected in both seismic and infrasound data during 4-8 February. Steam and low-level ash emissions likely occurred daily, though due to weather clouds they were only confirmed in satellite and webcam images during 1-2 and 5-8 February. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

26 January-1 February 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 25 January-1 February. Seismicity was elevated, characterized by periods of tremor and small earthquakes. Steam and low-level ash emissions likely occurred daily, though due to weather clouds they were only confirmed in webcam images during 25-26 and 29-31 January and 1 February. Numerous small explosions were recorded by local seismic and infrasound sensors during 29 January-1 February. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

19 January-25 January 2022 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi's North Cerberus cone continued during 18-25 January. Seismicity was elevated, characterized by periods of tremor and low-frequency earthquakes. Steam emissions were periodically visible in webcam images during 18-19 January and minor explosions were recorded by local seismic and infrasound sensors on 19 January. Steam and low-level ash emissions were visible in webcam and satellite images during 20-25 January. Daily explosions were recorded during 21-25 January, and elevated surface temperatures were identified in satellite data during 22-23 January. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

12 January-18 January 2022 Cite this Report

AVO reported that low-level eruptive activity and elevated seismicity at Semisopochnoi's North Cerberus cone continued during 12-18 January. A small explosion was recorded by local seismic and infrasound sensors during 12-13 January. The weather was mostly cloudy, though low-level ash clouds were occasionally visible in webcam images during 12-15 January. Steam emissions were visible in 15-16 January webcam images. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

5 January-11 January 2022 Cite this Report

AVO reported that low-level eruptive activity and elevated seismicity at Semisopochnoi's North Cerberus crater continued during 4-11 January. Almost daily explosions (one per day during 4-8 January and a few during 10-11 January) were recorded by the local seismic network. The weather was mostly cloudy, though ash and/or steam plumes were occasionally visible in webcam images. Low-level ash clouds were visible in webcam images during 10-11 January. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

29 December-4 January 2022 Cite this Report

AVO reported that low-level eruptive activity and elevated seismicity at Semisopochnoi's North Cerberus crater continued during 29 December 2021 to 4 January 2022. Minor ash-and-steam emissions were visible in webcam images during 28-29 December. Ash plumes observed in webcam and satellite images on 31 December rose to 1.5 km (5,000 ft) a.s.l. and drifted as far as 100 km NE. Ash emissions observed in webcam images during daylight hours on 1 and 2 January were being blown down the flank by high winds. Small explosions were detected in seismic data during 2-4 January, though cloud cover obscured views. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

22 December-28 December 2021 Cite this Report

AVO reported that low-level eruptive activity and elevated seismicity at Semisopochnoi's North Cerberus crater continued during 22-28 December. Small explosions were detected almost daily in seismic and infrasound data. Low-level ash-and-steam emissions were observed by webcams and in satellite images during 22-25 December, when weather conditions were clear. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

15 December-21 December 2021 Cite this Report

AVO reported that low-level eruptive activity and elevated seismicity at Semisopochnoi's North Cerberus crater continued during 15-21 December. Small daily explosions were detected in seismic and infrasound data. Low-level ash-and-steam emissions were observed daily by webcams and were occasionally identified in satellite images when weather conditions permitted. Elevated surface temperatures were also identified in satellite data during 15-17 and 19-20 December. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

8 December-14 December 2021 Cite this Report

AVO reported that low-level eruptive activity and elevated seismicity at Semisopochnoi's North Cerberus crater continued during 8-14 December. Several daily explosions were detected in seismic and infrasound data. Small ash clouds drifting at least 185 km SE were identified in satellite images during 8-9 December. Persistent steam emissions occasionally punctuated by ash emissions were visible in webcam views during 10-11 December, and ash emissions were visible the next day even though conditions were mostly cloudy. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

1 December-7 December 2021 Cite this Report

AVO reported that low-level eruptive activity and elevated seismicity at Semisopochnoi's North Cerberus crater continued during 1-7 December. Several daily explosions were detected in seismic and infrasound data. Weather clouds sometimes obscured satellite and webcam views of the volcano, though on most days low-level ash-and-steam plumes rising no higher than 1.5 km (5,000 ft) a.s.l. were visible mostly in webcam images. During 3-4 December a diffuse ash plume was identified in a satellite image drifting about 100 km E. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

24 November-30 November 2021 Cite this Report

AVO reported that eruptive activity at Semisopochnoi's North Cerberus crater continued during 24-30 November. Daily minor explosions were detected in seismic and infrasound data. Weather clouds obscured satellite and webcam views of the volcano on most days; small ash plumes rising no higher than 3 km (10,000 ft) a.s.l. were sometimes visible during breaks in the cloud cover but were likely emitted daily. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

17 November-23 November 2021 Cite this Report

AVO reported that eruptive activity at Semisopochnoi's North Cerberus crater continued during 17-23 November. Daily minor explosions were detected in seismic and infrasound data. Small ash plumes rising no higher than 3 km (10,000 ft) a.s.l. likely continued to be emitted, though weather clouds obscured satellite and webcam views of the volcano on most days. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

10 November-16 November 2021 Cite this Report

AVO reported that eruptive activity at Semisopochnoi's North Cerberus crater continued during 10-16 November. Daily minor explosions were detected in seismic and infrasound data. Daily ash plumes were visible in webcam and satellite data rising to 1.5-3.7 (5,000-12,000 ft) a.s.l., drifting NE, E, SE, and S, and dissipated quickly. Weather clouds sometimes obscured views. Ash plumes were typically dissipating within 50 km of the volcano. Minor ashfall on the island was sometimes visible. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

3 November-9 November 2021 Cite this Report

AVO reported that eruptive activity at Semisopochnoi's North Cerberus crater continued during 2-9 November. Daily tremor and minor explosions were detected in seismic and infrasound data. Several small low-level ash plumes were visible in webcam data rising to an estimated altitude of 1.5 (5,000 ft) a.s.l. during 2-3 November. Weather clouds obscured views during 4-7 November with the top of the cloud deck varying at altitudes of 3-6.1 km (10,000-20,000 ft) a.s.l.; ash plumes likely continued to be emitted though none rose above the cloud deck. Ash plumes were typically dissipating within 50 km of the volcano. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

27 October-2 November 2021 Cite this Report

AVO reported that eruptive activity at Semisopochnoi's North Cerberus crater continued during 26 October-2 November. Daily tremor and minor explosions were detected in seismic and infrasound data. Although weather clouds often prevented webcam and satellite views, intermittent, low-level ash emissions were visible during clear views, rising as high as 3.7 km (12,000 ft) a.s.l. and dissipating quickly. Sulfur dioxide emissions were detected during 26-27 and 30-31 October. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

20 October-26 October 2021 Cite this Report

AVO reported that eruptive activity at Semisopochnoi's North Cerberus crater continued during 19-26 October. Tremor and multiple minor explosions were detected daily in seismic and infrasound data. Although weather clouds often prevented webcam and satellite views, intermittent, low-level ash emissions were visible during clear weather rising to altitudes up to 3 km (10,000 ft a.s.l.); the farthest plume drifted 250 km on 19 October. Sulfur dioxide emissions were detected during 25-26 October. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

13 October-19 October 2021 Cite this Report

AVO reported that eruptive activity at Semisopochnoi's North Cerberus crater continued during 13-19 October. Seismicity remained elevated and daily minor explosions were detected in infrasound data. Although weather clouds often prevented webcam and satellite views, intermittent, low-level ash emissions were visible rising to altitudes up to 3 km (10,000 ft a.s.l.) and drifting E on 13 October. Ash plumes rose as high as 1.5 km (5,000 ft) a.s.l. on 18 October. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

6 October-12 October 2021 Cite this Report

AVO reported that eruptive activity at Semisopochnoi's North Cerberus crater continued during 6-12 October. Seismicity remained elevated and a few explosions per day were detected in infrasound data. Although weather clouds often prevented webcam and satellite views, discontinuous, low-level ash emissions were visible rising to altitudes up to 3 km (10,000 ft a.s.l.) and drifting E during 8-9 October. Low-level ash emissions were also visible in webcam images during 9-12 October. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

29 September-5 October 2021 Cite this Report

AVO reported that eruptive activity at Semisopochnoi's North Cerberus crater continued during 29 September-5 October. Seismicity remained elevated with intermittent explosion signals or bursts of activity likely from explosions. A few explosions were also detected in regional infrasound data during 2-5 October. Small ash clouds were observed almost daily in either webcam or satellite images rising to altitudes below 3 km (10,000 ft) a.s.l.; plumes drifted ENE on 2 October, then N and NW on 3 October. Sulfur dioxide plumes were possibly observed on a few days, though definitely on 3 October. Webcams located 5-6 km from the active vent recorded ashfall during 4-5 October. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

22 September-28 September 2021 Cite this Report

AVO reported that eruptive activity at Semisopochnoi's North Cerberus crater continued during 22-28 September. Seismicity remained elevated; daily explosions were recorded by seismic and infrasound networks. The frequency and intensity of ash emissions decreased during 21-22 September with occasional discrete ash clouds drifted W at altitudes of 3-4.6 km (10,000-15,000 ft) a.s.l. Sulfur dioxide plumes also drifted W. Occasional low-level ash emissions drifted NW, W, and SE during 22-26 September. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

15 September-21 September 2021 Cite this Report

AVO reported that eruptive activity at Semisopochnoi's North Cerberus crater continued during 15-21 September. Weather clouds obscured views of the volcano on most days. Several small daily explosions were recorded by local seismic stations; ash plumes were not visible in satellite images suggesting that they remained below 3 km (10,000 ft) a.s.l. and did not rise above the cloud deck. Overnight during 18-19 September small, discontinuous, low-level ash plumes were visible drifting 100 km SE. Ash emissions increased in frequency and intensity on 19 September. Ash clouds rose as high as 4.6 km (15,000 ft) a.s.l. and continued to drift 100 km SE. Sulfur dioxide emissions increased in the afternoon. AVO raised the Aviation Color Code to Red and the Volcano Alert Level to Warning at 2158. Explosions continued overnight and the next day, and ash plumes rose up to 4.6 km a.s.l. Plumes drifted 100 km NW. At 2012 on 20 September AVO lowered the Aviation Color Code to Orange noting the frequency of discrete explosions had decreased to a rate of about one per hour, though ash plumes from these events were still rising to 4.6 km a.s.l. and drifting NW. Sulfur dioxide plumes drifted N during 20-21 September.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

8 September-14 September 2021 Cite this Report

AVO reported that eruptive activity at Semisopochnoi's North Cerberus crater continued during 7-14 September. Seismicity was elevated and characterized by periods of continuous tremor. Short-lived explosions lasting several minutes were detected daily in infrasound data. Small ash clouds from the explosions rose 3-4.6 km (10,000-15,000 ft) and dissipated within two hours. Sulfur dioxide emissions were detected in satellite images at altitudes less than 3 km (10,000 ft) a.s.l., sometimes extending downwind for hundreds of kilometers. During 7-9 September periods of lower-altitude ash emissions interspersed with voluminous steam plumes were observed in web camera images moving horizontally by the wind and rising no higher than 1.5 km (5,000 ft) a.s.l. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

1 September-7 September 2021 Cite this Report

AVO reported that eruptive activity at Semisopochnoi's North Cerberus crater continued during 31 August-7 September. Multiple daily explosions were detected by seismic and infrasound networks. Ash-and-steam plumes from the explosions were sometimes confirmed in satellite and webcam images rising to altitudes lower than 3 km (10,000 ft) a.s.l., though during 6-7 September ash plumes rose as high as 4.6 km (15,000 ft) a.s.l. Local ashfall on the island was visible in satellite data. Sulfur dioxide emissions were detected in satellite images during 31 August-2 September and on 6 September. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

25 August-31 August 2021 Cite this Report

AVO reported that eruptive activity at Semisopochnoi's North Cerberus crater continued during 25-31 August. Multiple daily explosions were detected by seismic and infrasound networks. Ash-and-steam plumes from the explosions were sometimes confirmed in webcam images, in satellite data, and by nearby observers; plumes rose no higher than 3 km (10,000 ft) a.s.l. and produced local ashfall on the island. Daily sulfur dioxide emissions were identified in satellite images, drifting as far as 300 km NW during 29-31 August. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

18 August-24 August 2021 Cite this Report

AVO reported that multiple explosions and seismicity at Semisopochnoi's North Cerberus crater continued during 18-24 August. Low-level gas-and-ash emissions, including occasional sulfur dioxide emissions were detected in satellite and webcam data during 18-24 August and rose no higher than 3 km (10,000 ft) a.s.l.; views were often obscured due to weather. On 20 August minor ashfall deposits were reported; ashfall may have continued following explosive events during the rest of the week. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

11 August-17 August 2021 Cite this Report

AVO reported that an eruption at Semisopochnoi’s North Cerberus crater was ongoing during 11-17 August, characterized by daily explosions, ash plumes, and occasional sulfur dioxide emissions identified in satellite images. Multiple energetic explosions were detected daily in seismic and infrasound data, and strong seismic tremor was occasionally recorded. Daily steam-and-ash plumes identified in webcam and satellite data rose no higher than 3 km (10,000 ft) a.s.l. and drifted 200-300 km W. Daily ashfall likely occurred on the island. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

4 August-10 August 2021 Cite this Report

AVO reported that an eruption at Semisopochnoi’s North Cerberus crater was ongoing during 4-10 August, characterized by daily explosions, ash plumes, and sulfur dioxide emissions identified in satellite images. Multiple energetic explosions were detected almost daily in seismic and infrasound data. An ash cloud rose to 1.5-3 km (5,000-10,000 ft) a.s.l. and drifted 33 km NE during 3-4 August. Ash emissions were also visible in webcam images and photographed by an observer S of the island. An ash cloud was generated at 2123 on 4 August and was visible in satellite data until 0250 on 5 August, drifting 180 km SE over Gareloi and southern Tanaga Island. A small ash cloud at 2040 on 6 August drifted 50 km beyond the N crater vent and quickly dissipated. Another small ash cloud at 2103 caused ashfall on the webcam located 5 km NE about 30 minutes later. Ash-and-steam emissions were visible in webcam images until 2333. A large infrasound signal was recorded at 0653 on 8 August, though weather conditions prevented visual confirmation of emissions. A possible small ash cloud was visible in a satellite image at 0700. At least three separate explosions were recorded at 1714 on 8 August and 0339 and 0417 on 9 August. A small ash cloud at 900 m (3,000 ft) a.s.l. was visible in satellite images on the morning of 9 August. There were 10 explosions recorded during 9-10 August; small ash-and-steam emissions were visible in the webcam and small low-level plumes were occasionally identified in satellite data. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

28 July-3 August 2021 Cite this Report

AVO reported that seismicity at Semisopochnoi had increased in recent days. A notable increase at 0200 on 31 July prompted AVO to raise the Aviation Color Code to Orange and the Volcano Alert Level to Watch a few hours later. Additionally, increased infrasound recorded by local stations suggested low-level emissions. Weather clouds obscured satellite views, though a possible sulfur dioxide plume was detected. Steaming was visible in webcam images. Increased seismicity and elevated activity in infrasound data were recorded during 31 July-1 August, and multiple, discrete, energetic explosions were detected during 1-3 August. Weather clouds again obscured satellite views; steaming was visible in webcam views. Beginning at 1015 on 2 August the local networks recorded an explosion and an intense burst of seismic and acoustic tremor that lasted for about 15 minutes. A small ash cloud was visible in satellite images drifting 60 km N at altitudes of 1.5-3 km (5,000-10,000 ft) a.s.l. Ash emissions from the active vent in North Crater were visible in webcam views on the morning of 3 August.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

21 July-27 July 2021 Cite this Report

AVO lowered the Aviation Color Code to Yellow and the Volcano Alert Level to Advisory for Semisopochnoi on 22 July, noting that explosive activity had not been detected since 12 July. Seismicity continued to be elevated during 23-27 July. Robust steam emissions were visible in webcam views during 22-24 July and sulfur dioxide plumes were identified in satellite images on 23 July. Weather clouds obscured views during 24-27 July.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

14 July-20 July 2021 Cite this Report

AVO reported that unrest continued at Semisopochnoi during 13-20 July. Periods of low-level tremor and steam plumes from Mount Cerberus were occasionally recorded. Elevated surface temperatures at the N cone of Mount Cerberus were identified in satellite images during 13-14 July and robust steam-and-gas emissions were seen in webcam images. A plume with low amounts of sulfur dioxide drifting about 200 km N was identified in satellite data on 17 July. Sulfur dioxide emissions were also detected the next day. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

7 July-13 July 2021 Cite this Report

AVO reported that continuous volcanic tremor at Semisopochnoi began at 1200 on 12 July and explosive activity was recorded by the infrasound network. Emissions began at 1300 and lasted tens of minutes; a sulfur dioxide gas plume possibly containing ash was identified in satellite images drifting S at an altitude less than 1.5 km (5,000 ft) a.s.l. The Aviation Color Code was raised to Orange and the Volcano Alert Level was raised to Watch. Volcanic tremor decreased to low levels after several hours.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

16 June-22 June 2021 Cite this Report

AVO lowered Semisopochnoi’s Aviation Color Code to Yellow and the Volcano Alert Level to Advisory on 16 June, reporting that no explosions or ash emissions had been detected since 30 May and seismicity was relatively low. The report noted that elevated surface temperatures and sulfur dioxide emissions were recorded on several occasions during the previous few weeks, indicating continued volcanic unrest.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

9 June-15 June 2021 Cite this Report

AVO reported that seismic activity at Semisopochnoi was low during 9-15 June and no eruptive activity was detected in infrasound or satellite data. Elevated surface temperatures and steaming from the active vent were occasionally identified in satellite images. Sulfur dioxide emissions were visible in satellite data during 11-13 June. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

2 June-8 June 2021 Cite this Report

AVO reported that low-level unrest at Semisopochnoi continued during 2-8 June with seismicity occasionally above background levels. Steaming from Mount Cerberus was sometimes observed by field crews. Slightly elevated surface temperatures were identified in a few satellite infrared images during 4-7 June. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

26 May-1 June 2021 Cite this Report

AVO reported that an eruption at Semisopochnoi continued during 25 May-1 June. Steaming from the N crater at Mount Cerberus was visible in satellite images on 25 May. Radar data acquired between 15 and 27 May showed no morphological changes to the active vent. Tremor began to be recorded after AVO field engineers restored the satellite uplink to the seismic stations on 26 May. Field crews on 29 May observed low-level ash plumes rising to 2.4-3 km (8,000-10,000 ft) a.s.l., and small ash clouds drifting SW were visible in satellite images at around 0810 and 0920. Diffuse ash emissions were visible in satellite imagery at 1110. Strongly elevated surface temperatures were also identified in satellite data. Increased seismic tremor and accompanying low-level ash emissions began at 0945 on 30 May, with ash plumes drifting S at an altitude of 1.5 km (5,000 ft) a.s.l. at least through 1250. Elevated surface temperatures were detected during 30-31 May, likely from hot material on the crater floor. Seismic data showed low-level continuous tremor and occasional small discrete events. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

19 May-25 May 2021 Cite this Report

AVO reported that sulfur dioxide and steam plumes from Semisopochnoi were possibly visible in satellite images during 19-20 May. Satellite images on 21 May showed a sulfur dioxide gas plume drifting 160 km SE. Steam plumes rose from Cerberus on 24 May. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

12 May-18 May 2021 Cite this Report

AVO reported that weakly elevated surface temperatures and sulfur dioxide gas emissions at Semisopochnoi were identified in satellite images during 16-17 May. Several small explosions were recorded in infrasound data during 17-18 May; a small ash cloud was observed in a satellite image from 1521. Sulfur dioxide gas emissions and steaming from the active vents were identified in satellite data. The Aviation Color Code was raised to Orange and the Volcano Alert Level was raised to Watch on 18 May.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

5 May-11 May 2021 Cite this Report

AVO lowered the Aviation Color Code to Yellow and the Volcano Alert Level to Advisory for Semisopochnoi on 7 May, noting that no ash emissions had been observed nor explosions recorded since 26 April. Sulfur dioxide emissions were recorded on 1 and 5 May, signifying continued unrest. Steam plumes were visible rising from the N crater of Mt. Cerberus on 7 May.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

21 April-27 April 2021 Cite this Report

Eruptive activity at Semisopochnoi gradually increased during 21-22 April based on satellite and infrasound data; ash emissions were first seen in satellite images at 1800 on 21 April and were continuous at least through 1348 on 22 April. Plumes rose to 2.4 km (8,000 ft) a.s.l. and drifted 80 km S. Sulfur dioxide emissions were also detected in satellite images on 22 April. Cloud cover mostly obscured views during 23-24 April, though possible minor ash emissions were sometimes visible. Low-level ash plumes drifting S were occasionally identified in satellite images during 24-26 April. Dense weather clouds prevented views for most of 26 April and the next day. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

14 April-20 April 2021 Cite this Report

AVO reported that an eruption at Semisopochnoi continued during 14-20 April. Sulfur dioxide emissions were identified in satellite images during 13-14 April. An explosion was recorded by the regional infrasound network at 0417 on 14 April, though weather cloud cover as high as 4 km (13,000 ft) a.s.l. prevented satellite confirmation of an ash plume. Ash emissions began during the morning of 15 April and continued through the day, drifting more than 350 km SE at altitudes as high as 6 km (20,000 ft) a.s.l. They did not decrease in intensity, so at 1915 AVO raised the Aviation Color Code to Red and the Volcano Alert Level to Warning. Sustained ash emissions continued on 16 April, though the plume rose as high as 3 km (10,000 ft) a.s.l. and winds pushed it N. Explosions were detected through the night and early in the morning. Minor ash deposits around the volcano were visible. Eruptive activity declined during 16-17 April; one clear satellite view suggested that activity had declined or ceased. At 1249 on 17 April AVO lowered the Aviation Color Code to Orange and the Volcano Alert Level to Watch. Several explosions were recorded during 17-19 April though weather clouds obscured views. Volcanic plumes were visible just above the weather cloud deck (situated at 3 km (10,000 ft) a.s.l.) and drifted S during 19-20 April. A few darker (possibly ash rich) plumes were visible in satellite data at 0700 and 1150 on 19 April and on 20 April.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

7 April-13 April 2021 Cite this Report

AVO reported that low-level ash emissions from Semisopochnoi were visible in satellite images on 12 April along with a steam plume drifting E beyond the island. Additionally, new ash deposits extending SE at least to the coastline were also visible. The event was recorded by the regional infrasound network. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

31 March-6 April 2021 Cite this Report

AVO reported that a small plume from Semisopochnoi’s North Cerberus crater was visible in a satellite image on 31 March. Steam obscured the crater during the first part of 1 April. Later that day an ash plume was visible, and was followed by a sulfur dioxide plume an hour later. At 2200 a plume rose to an altitude below 3 km (10,000 ft) a.s.l. No activity was identified in partly cloudy images during 2-6 April. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

24 March-30 March 2021 Cite this Report

AVO reported that satellite data showed minor ash deposits on Semisopochnoi's flanks. Three explosions on 25 March (at 0142, 0330, and 0645) were recorded by regional infrasound sensors. Two more explosions were detected at 2130 on 26 March and at 0030 on 27 March; high weather cloud cover at 9.1 km (30,000 ft) a.s.l. prevented satellite confirmation; no ash was visible above the cloud deck during 25-27 March. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

17 March-23 March 2021 Cite this Report

AVO reported that satellite data showed minor ash deposits on Semisopochnoi’s flanks and a possible gas cloud on 14 March. Two small explosions on 19 March, at 0350 and 0534, were recorded by regional infrasound sensors and prompted AVO to raise the Aviation Color Code and the Volcano Alert Level to Orange and Watch, respectively. A small explosion at 0230 on 21 March was followed by a series of smaller explosions. A volcanic gas cloud was visible in satellite data during the previous day. Three small explosions were detected during 22-23 March, though high weather cloud cover, at 9.1 km (30,000 ft) a.s.l., prevented satellite confirmation; no ash was visible above the cloud deck.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

10 March-16 March 2021 Cite this Report

On 12 March AVO reported that although no ash plumes were observed at Semisopochnoi and nothing was detected in regional infrasound data during the previous week, ash deposits within Mount Cerberus’s N crater and extending 1.5 km on the flank were identified in satellite data. The Aviation Color Code and the Volcano Alert Level remained at Yellow/Advisory, respectively.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

17 February-23 February 2021 Cite this Report

AVO reported that no significant activity at Semisopochnoi was recorded after several ash deposits were visible in satellite images during 6-7 February. On 19 February the Aviation Color Code and the Volcano Alert Level were lowered to Yellow/Advisory, respectively.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

3 February-9 February 2021 Cite this Report

AVO reported that a 6 February satellite image of Semisopochnoi showed small ash deposits extending in a narrow strip less than 3 km N of North Cerberus Crater. The deposits were likely the result of a small explosions that occurred during the previous week. Steam emission obscured view of the summit crater. The Aviation Color Code and the Volcano Alert Level were raised to Yellow/Advisory. A second ash deposit, similar to the first, was visible in a satellite image on 7 February. This deposit extended at least 3 km NE of North Cerberus Crater. Weather clouds obscured views of the S side of the volcano. The report noted that the ash plumes associated with the deposits had not been observed; they likely rose less than 3 km (10,000 ft) a.s.l. and were short lived. The Aviation Color Code and the Volcano Alert Level were raised to Orange/Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

18 November-24 November 2020 Cite this Report

AVO reported that no signs of eruptive activity at Semisopochnoi had been recorded since mid-June. Seismicity had been elevated above background levels but declined to very low levels before 11 November, when the satellite the link to transmit the data failed. Both the Aviation Color Code and the Volcano Alert Level were lowered to Unassigned, reflecting the lack of available seismic data to detect unrest.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

1 April-7 April 2020 Cite this Report

On 1 April AVO reported that seismic, infrasound, and satellite data collected during the previous two weeks indicated no signs of eruptive activity at Semisopochnoi; the Aviation Color Code was lowered to Yellow and the Volcano Alert Level was lowered to Advisory. A crater lake and robust steam plume were both identified in recent satellite images.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

25 March-31 March 2020 Cite this Report

AVO reported that seismicity at Semisopochnoi remained elevated during 25-31 March and no surface activity was observed on most days, though weather clouds often prevented visual confirmation. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

18 March-24 March 2020 Cite this Report

AVO reported that activity at Semisopochnoi had decreased compared to the previous week. Seismicity remained elevated during 19-24 March and no surface activity was observed on most days, though weather clouds often prevented visual confirmation. A steam plume was visible in a satellite image on 21 March and weakly elevated surface temperatures were identified in one satellite image during 21-22 March. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

11 March-17 March 2020 Cite this Report

Seismicity at Semisopochnoi increased over a period of a few days and by 15 March was characterized by nearly continuous tremor and frequent signals indicating small explosions. The unrest prompted AVO to raise the Aviation Color Code to Orange and the Volcano Alert Level to Watch on 15 March. A robust steam plume rising from the N cone of Mount Cerberus and minor ash deposits around the crater rim were visible in a clear satellite image acquired on 16 March.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

26 February-3 March 2020 Cite this Report

No signs of eruptive activity at Semisopochnoi were detected in infrasound and satellite data over the previous several weeks, prompting AVO to lower the Aviation Color Code to Yellow and the Volcano Alert Level to Advisory on 26 February. A robust steam plume rose from the main vent that same day. Short bursts of tremor and earthquakes persisted during 26 February-3 March.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

19 February-25 February 2020 Cite this Report

AVO reported that intermittent, low-level tremor was detected at Semisopochnoi during 19-25 February. Brief periods of elevated tremor were recorded during 19-20 February. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

12 February-18 February 2020 Cite this Report

A series of explosions and tremor bursts at Semisopochnoi were detected by the seismic network beginning on 14 February, prompting AVO to raise the Aviation Color Code to Orange and the Volcano Alert Level to Watch. Tremor bursts lasting from three to twelve minutes every few hours were recorded the next day. Ash plumes were not visible, though a weather cloud deck persisted between 1.5 and 4.6 km (5,000 and 15,000 ft) a.s.l. Seismic data continued to record small explosions and tremor bursts lasting from six to ten minutes every few hours on 16 February, but by 17 February there were almost no events recorded. Weather clouds continued to obscure views of the volcano.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

8 January-14 January 2020 Cite this Report

On 9 January AVO lowered the Aviation Color Code for Semisopochnoi to Yellow and the Volcano Alert Level to Advisory, noting that explosions had not been detected since 19 December 2019. In addition, seismic tremor had last been recorded on 29 December 2019; seismicity subsequently declined but remained above background levels.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

18 December-24 December 2019 Cite this Report

AVO reported that ash plumes below 1.5 km (5,000 ft) a.s.l. from Semisopochnoi were identified in satellite images on 14 and 17 December; ash plumes on 17 December drifted about 15 km SE. During 17-20 December seismicity was characterized by tremor bursts and small explosions, though cloudy weather conditions prevented visual confirmation. Elevated seismicity was recorded on 21 December. Nothing significant was detected during 22-24 December. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

11 December-17 December 2019 Cite this Report

AVO reported that during 10-17 December activity at Semisopochnoi remained elevated, with bursts of tremor and small explosions detected in both seismic and infrasound data. An 80-km-long gas-and-steam plume possibly containing ash was visible in satellite images during 11-12 December. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

4 December-10 December 2019 Cite this Report

Strong tremor at Semisopochnoi was recorded by local seismic and regional infrasound networks beginning at 0026 on 7 December, heralding the start of an eruption and prompting AVO to raise the Aviation Color Code to Orange and the Volcano Alert Level to Watch. The meteorological cloud deck over the volcano was at around 3 km (10,000 ft) a.s.l.; no ash signals were detected above this altitude. A period of intermittent explosions was detected, and afterwards seismicity remained elevated at least through 9 December.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

18 September-24 September 2019 Cite this Report

AVO lowered the Volcano Alert Level for Semisopochnoi to Advisory and the Aviation Color Code to Yellow on 18 September noting a decrease in seismic activity over the past few weeks and an absence of tremor since 15 September. Low-levels sulfur dioxide emissions persisted, and the crater lake continued to fill with water.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

4 September-10 September 2019 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi possibly continued during 4-10 September. Seismic activity was relatively minor and characterized by intermittent low-frequency earthquakes. Satellite images were mostly cloudy, though a low-altitude steam plume drifting 50 km S was visible on 8 September. The Volcano Alert Level remained at Watch and the Aviation Color Code remained at Orange.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

28 August-3 September 2019 Cite this Report

AVO reported that low-level eruptive activity at Semisopochnoi possibly continued during 28 August-3 September. Seismic activity was relatively minor and characterized by intermittent low-frequency earthquakes. Minor tremor was detected until 30 August. Satellite images were obscured by clouds. The Volcano Alert Level remained at Watch and the Aviation Color Code remained at Orange.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

21 August-27 August 2019 Cite this Report

AVO reported that during 17-23 August seismicity at Semisopochnoi remained elevated and was characterized by periods of continuous tremor and discrete low-frequency earthquakes; seismic data went offline starting sometime on 17 August, though was available by around 22 August. Ground-coupled airwaves, indicative of explosive activity, were sometimes recorded in seismic data; an infrasound signal was recorded during 23-24 August. Cloudy weather often prevented satellite views of the volcano, though a steam plume was visible on 18 August and sulfur dioxide emissions were detected during 21-22 August. The Volcano Alert Level remained at Watch and the Aviation Color Code remained at Orange.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

14 August-20 August 2019 Cite this Report

AVO reported that during 14-17 August seismicity at Semisopochnoi remained elevated and was characterized by periods of continuous tremor and discrete low-frequency earthquakes; seismic data became unavailable starting sometime on 17 August. Cloudy weather often prevented satellite views of the volcano, however some recent clear views indicated that the N cone crater had a smooth featureless area, indicating water or tephra at an elevation several meters below the previous floor. The Volcano Alert Level remained at Watch and the Aviation Color Code remained at Orange.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

7 August-13 August 2019 Cite this Report

AVO reported that during 7-13 August seismicity at Semisopochnoi remained elevated and was characterized by periods of continuous tremor and discrete low-frequency earthquakes. No unusual activity was observed in satellite images, though views were often cloudy. The Volcano Alert Level remained at Watch and the Aviation Color Code remained at Orange.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

31 July-6 August 2019 Cite this Report

AVO reported that during 31 July-6 August seismicity at Semisopochnoi remained elevated and was characterized by periods of weak, continuous tremor and discrete low-frequency earthquakes. Satellite images were mostly cloudy, though a possible steam plume was visible during 5-6 August. The Volcano Alert Level remained at Watch and the Aviation Color Code remained at Orange.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

24 July-30 July 2019 Cite this Report

On 24 July AVO reported that satellite data from the previous week indicated that the 100-m-wide crater lake in the N cone of Semisopochnoi’s Cerberus three-cone cluster was gone, and a new shallow inner crater about 80 m in diameter had formed on the crater floor. The lake had persisted since January 2019. Seismicity during 25-30 July was characterized by periods of continuous tremor, low-frequency earthquakes, and small explosion signals. Small steam plumes were visible in periodic, cloud-free satellite images, along with minor sulfur dioxide emissions.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

17 July-23 July 2019 Cite this Report

AVO reported that data from local seismic and infrasound sensors likely detected a small explosion at Semisopochnoi on 16 July. No ash was visible in cloudy satellite images although none was expected from an explosion of its size. A small plume drifted 18 km from the vent but had no indication of ash. A strong tremor signal was recorded at 2339 on 17 July and an infrasound signal was detected from an array located 260 km E on Adak Island. The event likely produced ash emissions, though none were visible above the cloud deck at 3 km (10,000 ft) a.s.l. Seismic activity continued to increase. On 18 July a short-lived, low-level eruption prompted AVO to raise the Aviation Color Code to Orange (the second highest level on a four-color scale) and the Volcano Alert Level to Watch (the second highest level on a four-level scale). A low-level plume was visible in occasional cloud-free satellite images. Seismic activity decreased abruptly that night and ground-coupled airwaves stopped being detected on adjacent islands, suggesting that the eruption had paused or ended. Seismic activity remained low at least through 21 July.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

3 July-9 July 2019 Cite this Report

An increase in seismicity above background levels on 4 July at Semisopochnoi prompted AVO to raise the Aviation Color Code to Orange (the second highest level on a four-color scale) and the Volcano Alert Level to Watch (the second highest level on a four-level scale). Elevated seismicity continued through at least 9 July. No eruptive activity was detected in regional infrasound data, and cloudy conditions obscured satellite views of the volcano. [Correction: the Aviation Color Code was raised to Yellow and the Volcano Alert Level was raised to Advisory.]

Source: US Geological Survey Alaska Volcano Observatory (AVO)

21 November-27 November 2018 Cite this Report

AVO reported that no evidence of activity at Semisopochnoi had been detected since an explosion was recorded in infrasound data on 31 October. The satellite link for transmitting seismic data failed on 1 November, though no activity was observed in satellite or infrasound data since then. As a result, the Aviation Color Code was lowered to Yellow and Volcano Alert Level was lowered to Advisory (both are the second lowest levels on four-level scales) on 21 November.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

7 November-13 November 2018 Cite this Report

AVO reported that three possible small explosions at Semisopochnoi were detected in infrasound data between 1951 and 2004 on 9 November. No associated ash clouds were observed in partly cloudy satellite images, and no other activity was noted during 7-11 November also in partly cloudy images. Images were cloudy during 12-13 November. The Aviation Color Code remained at Orange and Volcano Alert Level remained at Watch (both are the second highest levels on four-level scales).

Source: US Geological Survey Alaska Volcano Observatory (AVO)

31 October-6 November 2018 Cite this Report

AVO reported that two small explosions at Semisopochnoi were detected in seismic and infrasound data on 31 October. Intermittent seismic tremor was recorded on 1 November but later that day the satellite link that transmits seismic data failed. Weather clouds obscured views of the volcano during 31 October-4 November. Nothing was observed in satellite data during 5-6 November. The Aviation Color Code remained at Orange (the second highest level on a four-color scale) and Volcano Alert Level remained at Watch (the second highest level on a four-level scale).

Source: US Geological Survey Alaska Volcano Observatory (AVO)

24 October-30 October 2018 Cite this Report

AVO reported that an eruptive event at Semisopochnoi began at 2047 on 25 October based on seismic data; strong volcanic tremor lasted about 20 minutes and was followed by 40 minutes of weak tremor pulses. A weak infrasound signal was detected by instruments on Adak Island (260 km SE). The Aviation Color Code was raised to Orange (the second highest level on a four-color scale) and Volcano Alert Level was raised to Watch (the second highest level on a four-level scale). A dense meteorological cloud deck prevented observations below 3 km (10,000 ft) a.s.l.; a diffuse cloud was observed in satellite data rising briefly above the cloud deck, though it was unclear if it was related to eruptive activity. Tremor ended after the event, and seismicity returned to low levels.

Small explosions were detected by the seismic network at 2110 and 2246 on 26 October and 0057 and 0603 on 27 October. No ash clouds were identified in satellite data, but the volcano was obscured by high meteorological clouds. Additional small explosions were detected in seismic and infrasound data during 28-29 October; no ash clouds were observed in partly-cloudy-to-cloudy satellite images.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

10 October-16 October 2018 Cite this Report

On 11 October AVO reported that satellite data of Semisopochnoi indicated partial erosion of a tephra cone in the crater of Cerberus’s N cone. A crater lake about 90 m in diameter filled the vent. The data also suggested that the vent had not erupted since 1 October. Seismicity remained elevated and above background levels. The next day AVO lowered the Aviation Color Code to Yellow and the Volcano Alert Level to Advisory, noting the recent satellite data results and lack of tremor recorded during the previous week.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

3 October-9 October 2018 Cite this Report

AVO reported that during 3-9 October seismicity at Semisopochnoi remained elevated, with intermittent bursts of tremor. No volcanic activity was detected in infrasound or satellite data. The Aviation Color Code (ACC) remained at Yellow and Volcano Alert Level (VAL) remained at Advisory.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

26 September-2 October 2018 Cite this Report

AVO reported that during 19-25 September seismicity at Semisopochnoi remained elevated, alternating between periods of continuous and intermittent bursts of tremor. Tremor bursts at 1319 on 21 September and at 1034 on 22 September produced airwaves detected on a regional infrasound array on Adak Island; no ash emissions were identified above the low cloud deck in satellite data, and the infrasound detections likely reflected an atmospheric change instead of volcanic activity. The Aviation Color Code (ACC) remained at Yellow and Volcano Alert Level (VAL) remained at Advisory.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

19 September-25 September 2018 Cite this Report

AVO reported that during 19-25 September seismicity at Semisopochnoi remained elevated, alternating between periods of continuous and intermittent bursts of tremor. Tremor bursts at 1319 on 21 September and at 1034 on 22 September produced airwaves detected on a regional infrasound array on Adak Island; no ash emissions were identified above the low cloud deck in satellite data, and the infrasound detections likely reflected an atmospheric change instead of volcanic activity. The Aviation Color Code (ACC) remained at Yellow and Volcano Alert Level (VAL) remained at Advisory.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

12 September-18 September 2018 Cite this Report

On 16 September AVO raised the Aviation Color Code (ACC) for Semisopochnoi to Yellow and Volcano Alert Level (VAL) to Advisory after increased seismicity was detected at 0831. Retrospective analysis of satellite data acquired on 10 September revealed small ash deposits on the N flank of Mount Cerberus, possibly associated with two bursts of tremor recorded on 8 September. This new information coupled with intensifying seismicity and a strong tremor signal recorded at 1249 on 17 September prompted AVO to raise the ACC to Orange and the VAL to Watch. Seismicity remained elevated on 18 September with nearly constant tremor being recorded by local sensors.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

27 May-2 June 2015 Cite this Report

A decline in seismicity at Semisopochnoi over the previous few months, and no activity observed in satellite images, prompted AVO to lower the Aviation Color Code and the Volcano Alert Level to Unassigned on 28 May. Increased seismicity had been detected in January.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

1 April-7 April 2015 Cite this Report

On 25 March AVO reported that seismicity at Semisopochnoi that had begun in January continued and had increased over the previous few days. Brief periods of tremor were also detected. AVO raised the Aviation Color Code to Yellow and the Volcano Alert Level to Advisory. The elevated seismicity, characterized by discrete fairly small earthquakes beneath the center of the island, continued to be detected through 7 April.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

18 June-24 June 2014 Cite this Report

AVO reported that the earthquake swarm that had started at Semisopochnoi on 9 June continued until 23 June. No eruptive activity was indicated. The Aviation Color Code remained at Yellow and the Volcano Alert Level remained at Advisory.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

11 June-17 June 2014 Cite this Report

AVO reported that an earthquake swarm at Semisopochnoi started at 1000 on 9 June and escalated at 1200 on 12 June. The continuation of the anomalous activity prompted AVO to raise the Aviation Color Code to Yellow and the Volcano Alert Level to Advisory on 13 June. The earthquake swarm was continuing as of 17 June. Five of the six seismic stations on the volcano were operational.

Source: US Geological Survey Alaska Volcano Observatory (AVO)

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This compilation of synonyms and subsidiary features may not be comprehensive. Features are organized into four major categories: Cones, Craters, Domes, and Thermal Features. Synonyms of features appear indented below the primary name. In some cases additional feature type, elevation, or location details are provided.

Synonyms
Tserber
Cones
Feature Name	Feature Type	Elevation	Latitude	Longitude
Anvil Peak	Cone	1221 m	51° 59' 0" N	179° 35' 13" E
Lakeshore Cone	Cone	178 m	51° 57' 11" N	179° 37' 23" E
Pochnoi	Shield volcano
Ragged Top	Cone	904 m	51° 55' 55" N	179° 40' 19" E
Sugarloaf Peak	Stratovolcano	856 m	51° 54' 0" N	179° 38' 0" E
Three-Quarter Cone	Cone	520 m	51° 57' 0" N	179° 32' 53" E
Young     Cerberus	Stratovolcano	774 m	51° 56' 10" N	179° 35' 35" E

Basic Data Volcano Number Last Known Eruption Elevation Latitude Longitude 311060 2023 CE 1,221 m / 4,006 ft 51.93°N 179.58°E Morphology Volcano Landform Composite Volcano Types Stratovolcano Caldera Cone(s) Rock Types Major Basalt / Picro-Basalt Andesite / Basaltic Andesite Dacite Trachyandesite / Basaltic Trachyandesite Tectonic Setting Subduction zone Intermediate crust (15-25 km) Population Within 5 km Within 10 km Within 30 km Within 100 km 0 0 0 0

Geological Summary Semisopochnoi, the largest subaerial volcano of the western Aleutians, is 20 km wide at sea level and contains an 8-km-wide caldera. It formed as a result of collapse of a low-angle, dominantly basaltic volcano following the eruption of a large volume of dacitic pumice. The high point of the island is Anvil Peak, a double-peaked late-Pleistocene cone that forms much of the island's northern part. The three-peaked Mount Cerberus (renamed Mount Young in 2023) was constructed within the caldera during the Holocene. Each of the peaks contains a summit crater; lava flows on the N flank appear younger than those on the south side. Other post-caldera volcanoes include the symmetrical Sugarloaf Peak SSE of the caldera and Lakeshore Cone, a small cinder cone at the edge of Fenner Lake in the NE part of the caldera. Most documented eruptions have originated from Young, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone could have been recently active. This volcano is located within the Aleutian Islands, a UNESCO Biosphere Reserve property. References The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography. Coats R R, 1950. Volcanic activity in the Aleutian Arc. U S Geol Surv Bull, 974-B: 35-47. Coats R R, 1959. Geologic reconnaissance of Semisopochnoi Island western Aleutian Islands Alaska. U S Geol Surv Bull, 1028-O: 477-519. Delong S E, Perfit M R, McCulloch M T, Ach J, 1985. Magmatic evolution of Semisopochnoi Island, Alaska: trace-element and isotopic constraints. J Geol, 93: 609-618. Henning R A, Rosenthal C H, Olds B, Reading E (eds), 1976. Alaska's volcanoes, northern link in the ring of fire. Alaska Geog, 4: 1-88. IAVCEI, 1973-80. Post-Miocene Volcanoes of the World. IAVCEI Data Sheets, Rome: Internatl Assoc Volc Chemistry Earth's Interior. Miller T P, McGimsey R G, Richter D H, Riehle J R, Nye C J, Yount M E, Dumoulin J A, 1998. Catalogue of the historically active volcanoes of Alaska. U S Geol Surv Open-File Rpt, 98-582: 1-104. Motyka R J, Liss S A, Nye C J, Moorman M A, 1993. Geothermal resources of the Aleutian arc. Alaska Div Geol Geophys Surv, Prof Rpt, no 114, 17 p and 4 map sheets. Myers J D, 1994. The Geology, Geochemistry and Petrology of the recent Magmatic Phase of the Central and Western Aleutian Arc. Unpublished manuscript, unpaginated. Smith R L, Shaw H R, Luedke R G, Russell S L, 1978. Comprehensive tables giving physical data and thermal energy estimates for young igneous systems of the United States. U S Geol Surv Open-File Rpt, 78-925: 1-25.

Eruptive History

There is data available for 6 confirmed Holocene eruptive periods.

Deformation History

There is data available for 1 deformation periods. Expand each entry for additional details.

Emission History

There is no Emissions History data available for Semisopochnoi.

Photo Gallery

A 7 June 1792, sketch from the east shows Semisopochnoi Island with three "smoking" volcanic cones. The 20-km-wide Semisopochnoi volcano is truncated by an 8-km-wide caldera. Three post-caldera cones, Cerebus, Lakeshore Cone, and Sugarloaf may have had eruptions in historical time. It is not certain whether the "smoke" plumes seen in this somewhat stylized sketch depict an ash eruption, or merely increased steaming.

From the collection of Maurice and Katia Krafft. An aerial view of the SE coast of Semisopochnoi Island shows Sugarloaf peak (left-center) with a flank cone (foreground) that was the source of one of the more recent lava flows. Sugarloaf erupted outside the southern margin of an 8-km-wide caldera on Semisopochnoi. Mount Cerberus was constructed within the caldera and is on the left horizon. Pre-caldera Ragged Top is to the right.

Photo by U.S. Navy (published in U.S. Geological Survey Bulletin 1028-O). The western slopes of Sugarloaf Peak (left) are seen here on Semisopochnoi Island. One of the largest subaerial volcanoes of the western Aleutians, Semisopochnoi is 20 km wide at sea level and contains an 8-km-wide caldera. Mount Cerberus was constructed within the caldera during the Holocene, along with Sugarloaf Peak outside the caldera to the SSE. Many documented historical eruptions originated from Cerberus.

Photo by Steve Ebbert, 1997 (U.S. Fish and Wildlife Service).

GVP Map Holdings

Maps are not currently available due to technical issues.

Smithsonian Sample Collections Database

The following 16 samples associated with this volcano can be found in the Smithsonian's NMNH Department of Mineral Sciences collections, and may be availble for research (contact the Rock and Ore Collections Manager). Catalog number links will open a window with more information.

Catalog Number	Sample Description	Lava Source	Collection Date
NMNH 118209-1	Basalt	--	12 Sep 2015
NMNH 118209-10	Scoria	--	12 Sep 2015
NMNH 118209-11	Lapilli tephra	--	12 Sep 2015
NMNH 118209-12	Scoria	--	12 Sep 2015
NMNH 118209-13	Lapilli tephra	--	12 Sep 2015
NMNH 118209-14	Scoriaceous tephra	--	12 Sep 2015
NMNH 118209-15	Scoriaceous tephra	--	12 Sep 2015
NMNH 118209-16	Scoria	--	12 Sep 2015
NMNH 118209-2	Basalt	--	--
NMNH 118209-3	Lapilli tephra	--	12 Sep 2015
NMNH 118209-4	Basalt	--	--
NMNH 118209-5	Basalt	--	12 Sep 2015
NMNH 118209-6	Fine ash	--	12 Sep 2015
NMNH 118209-7	Fine ash	--	12 Sep 2015
NMNH 118209-8	Pyroclastic flow	--	12 Sep 2015
NMNH 118209-9	Pyroclastic flow	--	12 Sep 2015

External Sites

Alaska Volcano Observatory (Link to Semisopochnoi)	The Alaska Volcano Observatory (AVO) is a joint program of the U.S. Geological Survey (USGS), the Geophysical Institute of the University of Alaska Fairbanks (UAFGI), and the State of Alaska Division of Geological and Geophysical Surveys (ADGGS). AVO was formed in 1988, and uses federal, state, and university resources to monitor and study Alaska's hazardous volcanoes, to predict and record eruptive activity, and to mitigate volcanic hazards to life and property.
Copernicus Browser	The Copernicus Browser replaced the Sentinel Hub Playground browser in 2023, to provide access to Earth observation archives from the Copernicus Data Space Ecosystem, the main distribution platform for data from the EU Copernicus missions.
MIROVA	Middle InfraRed Observation of Volcanic Activity (MIROVA) is a near real time volcanic hot-spot detection system based on the analysis of MODIS (Moderate Resolution Imaging Spectroradiometer) data. In particular, MIROVA uses the Middle InfraRed Radiation (MIR), measured over target volcanoes, in order to detect, locate and measure the heat radiation sourced from volcanic activity.
MODVOLC Thermal Alerts	Using infrared satellite Moderate Resolution Imaging Spectroradiometer (MODIS) data, scientists at the Hawai'i Institute of Geophysics and Planetology, University of Hawai'i, developed an automated system called MODVOLC to map thermal hot-spots in near real time. For each MODIS image, the algorithm automatically scans each 1 km pixel within it to check for high-temperature hot-spots. When one is found the date, time, location, and intensity are recorded. MODIS looks at every square km of the Earth every 48 hours, once during the day and once during the night, and the presence of two MODIS sensors in space allows at least four hot-spot observations every two days. Each day updated global maps are compiled to display the locations of all hot spots detected in the previous 24 hours. There is a drop-down list with volcano names which allow users to 'zoom-in' and examine the distribution of hot-spots at a variety of spatial scales.
WOVOdat    Single Volcano View    Temporal Evolution of Unrest    Side by Side Volcanoes	WOVOdat is a database of volcanic unrest; instrumentally and visually recorded changes in seismicity, ground deformation, gas emission, and other parameters from their normal baselines. It is sponsored by the World Organization of Volcano Observatories (WOVO) and presently hosted at the Earth Observatory of Singapore. GVMID Data on Volcano Monitoring Infrastructure The Global Volcano Monitoring Infrastructure Database GVMID, is aimed at documenting and improving capabilities of volcano monitoring from the ground and space. GVMID should provide a snapshot and baseline view of the techniques and instrumentation that are in place at various volcanoes, which can be use by volcano observatories as reference to setup new monitoring system or improving networks at a specific volcano. These data will allow identification of what monitoring gaps exist, which can be then targeted by remote sensing infrastructure and future instrument deployments.
Volcanic Hazard Maps	The IAVCEI Commission on Volcanic Hazards and Risk has a Volcanic Hazard Maps database designed to serve as a resource for hazard mappers (or other interested parties) to explore how common issues in hazard map development have been addressed at different volcanoes, in different countries, for different hazards, and for different intended audiences. In addition to the comprehensive, searchable Volcanic Hazard Maps Database, this website contains information about diversity of volcanic hazard maps, illustrated using examples from the database. This site is for educational purposes related to volcanic hazard maps. Hazard maps found on this website should not be used for emergency purposes. For the most recent, official hazard map for a particular volcano, please seek out the proper institutional authorities on the matter.
IRIS seismic stations/networks	Incorporated Research Institutions for Seismology (IRIS) Data Services map showing the location of seismic stations from all available networks (permanent or temporary) within a radius of 0.18° (about 20 km at mid-latitudes) from the given location of Semisopochnoi. Users can customize a variety of filters and options in the left panel. Note that if there are no stations are known the map will default to show the entire world with a "No data matched request" error notice.
UNAVCO GPS/GNSS stations	Geodetic Data Services map from UNAVCO showing the location of GPS/GNSS stations from all available networks (permanent or temporary) within a radius of 20 km from the given location of Semisopochnoi. Users can customize the data search based on station or network names, location, and time window. Requires Adobe Flash Player.
DECADE Data	The DECADE portal, still in the developmental stage, serves as an example of the proposed interoperability between The Smithsonian Institution's Global Volcanism Program, the Mapping Gas Emissions (MaGa) Database, and the EarthChem Geochemical Portal. The Deep Earth Carbon Degassing (DECADE) initiative seeks to use new and established technologies to determine accurate global fluxes of volcanic CO2 to the atmosphere, but installing CO2 monitoring networks on 20 of the world's 150 most actively degassing volcanoes. The group uses related laboratory-based studies (direct gas sampling and analysis, melt inclusions) to provide new data for direct degassing of deep earth carbon to the atmosphere.
Large Eruptions of Semisopochnoi	Information about large Quaternary eruptions (VEI >= 4) is cataloged in the Large Magnitude Explosive Volcanic Eruptions (LaMEVE) database of the Volcano Global Risk Identification and Analysis Project (VOGRIPA).
EarthChem	EarthChem develops and maintains databases, software, and services that support the preservation, discovery, access and analysis of geochemical data, and facilitate their integration with the broad array of other available earth science parameters. EarthChem is operated by a joint team of disciplinary scientists, data scientists, data managers and information technology developers who are part of the NSF-funded data facility Integrated Earth Data Applications (IEDA). IEDA is a collaborative effort of EarthChem and the Marine Geoscience Data System (MGDS).