On February 6, 2021, high-resolution satellite images showed a small ash deposit extending less than 3 km to the north from North Cerberus Crater on Semisopochnoi Island. Steam emissions obscured views into the crater. There is no evidence for continuing activity, but observation of a new deposit suggests renewed unrest, and the Alaska Volcano Observatory increased the Aviation Color Code to YELLOW and the Volcano Alert Level to ADVISORY. On February 7, 2021, high-resolution satellite imagery showed a second small ash deposit extending at least 3 km to the northeast from North Cerberus Crater on Semisopochnoi Island, similar to the first observed on February 6. In response, AVO raised the Aviation Color Code and Volcano Alert Level to ORANGE/WATCH. On February 19, citing no further significant volcanic activity, AVO lowered Semisopochnoi to YELLOW/ADVISORY.
March 19, 2021, regional infrasound sensors detected a small explosion from Semisopochnoi volcano at 11:50 UTC (03:50 AKDT), indicating an increase in eruptive activity. As a result, AVO raised the Aviation Color Code to ORANGE and the Volcano Alert Level to WATCH. Cloudy weather prevented satellite views.
April 16, 2021, satellite data suggested ash emissions from Semisopochnoi began early that morning and continued for several hours. The ash cloud extended up to 217 mi (350 km) southeast of the volcano with heights up to 20,000 feet (6 km) above sea level. AVO raised the Aviation Color Code and Volcano Alert Level to RED/WARNING. By April 17, eruptive activity declined and AVO lowered the Aviation Color Code and Volcano Alert Level to ORANGE/WATCH.
April 22, eruptive activity again increased, and satellite views showed a continuous plume extending 50 mi (80 km) to the south at approximately 8000 ft a.m.s.l.. A regional infrasound network detected the increase in activity.
On May 7, 2021, citing no ash emissions or explosions detected from Semisopochnoi since April 26, AVO lowered the Aviation Color Code and Volcano Alert Level to YELLOW/ADVISORY.
On May 19, 2021, several small explosions were detected in regional infrasound data and a small ash cloud was observed in a satellite image from 15:21 (23:21 UTC) on May 17. As a result, AVO raised the Aviation Color Code and Volcano Alert Level to ORANGE/WATCH. Low-level eruptive activity continued, including sulfur dioxide emissions and low-level ash clouds under 10,000 ft.
On June 16, 2021, AVO lowered the Aviation Color Code and Volcano Alert Level at Semisopochnoi to YELLOW/ADVISORY, due to decreasing seismicity and no ash emissions or explosions since May 30.
On July 12, 2021, the Aviation Color Code and Volcano Alert Level were increased to ORANGE/WATCH due to the onset of continuous seismic tremor, the detection of explosive activity on local infrasound sensors, and observations in satellite data of a small volcanic cloud. The cloud likely contained volcanic gas with minor amounts of volcanic ash, rose to an altitude of less than 5,000 ft asl, and quickly dispersed. Seismic and infrasound data show that the main pulse of explosive activity was tens of minutes in duration. After that small explosive event, seismicity declined to intermittent and short bursts of tremor.
In late July, seismic activity at Semisopochnoi increased, with a marked increase on July 31. Activity was also detected on local infrasound stations, suggesting possible low-level emissions. In response, AVO raised the Aviation Color Code and Volcano Alert Level to ORANGE/WATCH.
Throughout August and September, numerous explosions and small ash clouds reached 5-13,000 ft and drifted 50-185 miles. Many of these ash emissions were also observed in the two new webcameras installed by AVO field crews in June, 2021. On September 20, AVO briefly raised the aviation color code and volcano alert level from ORANGE/WATCH to RED/WARNING, based on an increase in the intensity of ash emissions. Satellite images showed an ash cloud reaching 15,000 ft and extending 60 mi to the southeast. By September 20, activity had decreased to discrete explosions about once per hour and AVO lowered the alert status for Semisopochnoi to ORANGE/WATCH. As of late November, 2021, the eruption at Semisopochnoi continues with small explosions producing low-level ash clouds, often multiple times per day. Most ash emissions rise to less than 10,000 ft asl and dissipate within 30 miles of the volcano, and some may result in minor ash deposits within the vicinity of the active north crater.