Semisopochnoi reported activity

Event Name : Semisopochnoi 2019/7

Start:	July 18, 2019	Observed
Stop:	June 15, 2020 ± 7 Days	Observed

Description: On July 4, 2019 (AKDT; 06:51 July 7, 2019, UTC), the Alaska Volcano Observatory raised the Aviation Color Code to YELLOW and Alert Level to ADVISORY at Semisopochnoi, due to an increase in seismicity persisting at least eight hours. At the time of the color code change, no explosive activity had been detected on the Adak infrasound array and clouds obscured satellite views of the volcano.

On July 18, AVO raised the color code to ORANGE and the volcano alert level to WATCH, citing "Elevated seismicity that began yesterday has increased overnight. A stronger tremor signal recorded at 23:39 AKDT / 07:39 UTC also produced an infrasound signal on an infrasound array 260 km (161 miles) east on Adak island. This event likely produced ash emissions, and at the current level of continued unrest similar events could occur with little to no warning. The meteorological cloud deck has been around 10,000' (3000 m) asl over Semisopochnoi and no ash signals have been detected above this height. A small plume extending 18 km (11 miles) from the Cerberus vent was visible in satellite data from yesterday, but did not contain an ash signal."

On September 18, 2019, AVO lowered the Aviation Color Code and Volcano Alert Level to YELLOW/ADVISORY, citing declining seismicity. Low level SO2 observations continued to be observed when conditions permitted.

On December 7, 2019, AVO detected an eruption signature of strong tremor in local seismic and in regional infrasound networks on Adak. The event started at 00:26 AKST/09:26 UTC. AVO raised the color code from YELLOW to ORANGE in response. Seismic tremor and sporadic explosion signals continued through the following week, along with a steam and gas plume that possibly contained volcanic ash on December 11 and 12, 2019. Sporadic low level eruptive activity continued through December 19, 2019. On January 9, 2020, citing no explosive activity since December 19, 2019 and decreasing seismic tremor, AVO lowered the color code from ORANGE to YELLOW.

On February 15, 2020, AVO again detected a series of small explosion bursts from seismic data, and raised the Aviation Color Code and Volcano Alert Level to ORANGE/WATCH. Although seismic activity remained above background, clear satellite views of Semisopochnoi on February 25, 2020, showed no signs of eruptive activity and AVO lowered the Aviation Color Code and Volcano Alert Level to YELLOW/ADVISORY on February 26, 2020. Seismic unrest continued, and by March 15 was characterized by nearly continuous tremor and frequent small explosion signals. AVO raised the Aviation Color Code and Volcano Alert Level to ORANGE/WATCH. Small explosions continued for several days, and clear satellite images showed a robust steam plume and minor ash deposits around North Cerebus' crater rim. Activity has since subsided. No explosions have been recorded since March 17, 2020, and no surface activity has been seen in satellite data, though the volcano was mostly obscured by clouds. On April 1, 2020, citing no further signs of eruptive activity at Semisopochnoi in the past two weeks, AVO lowered the Aviation Color Code and Volcano Alert Level to YELLOW/ADVISORY. Seismicity was at low levels, characterized by occasional small earthquakes. In early April, a robust steam and gas plume containing detectable sulfur dioxide was seen in satellite data.

Unrest continued at Semisopochnoi throughout the summer of 2020. In June, there were periods of moderate seismic tremor, at times accompanied by ground-coupled airwaves. In mid-June, high-resolution satellite images showed steaming from the North Cerebus crater, along with a light ash deposit near the crater. Periods of seismic tremor and observed sulfur-dioxide plumes continued to be observed in June and July. Seismicity declined in August, 2020, and remained low through the fall. On November 20, 2020, citing no eruptive activity since mid-June, AVO lowered the Aviation Color Code and Volcano Alert Level to UNASSIGNED, because AVO's satellite link for transmitting data failed on November 11,