Event Name : Semisopochnoi 2018/09
|Start:||September 16, 2018 ||Observed|Description:
AVO raised the Aviation Color Code and Volcano Alert Level at Semisopochnoi to YELLOW/ADVISORY on September 16, 2018, citing an increase in seismicity, beginnnig at 16:31 UTC (08:31 AKDT). On September 17, 2018, strong tremor increased at 20:49 UTC (12:49 AKDT), suggesting ash emissions may be occurring or imminent. AVO raised the Aviation Color Code and Volcano Alert Level to ORANGE/WATCH at 21:33 UTC (13:33 AKDT). A clear satellite view from September 10 revealed a small ash deposit on the flanks of the north cone of Mount Cerberus, which may have resulted from two minor seismic tremor bursts recorded on September 8.
Unrest and intermittent sesimic tremor continued at Semisopochnoi throughout September 2018. Some energetic events also produced infrasound recorded on the Adak infrasound arry (257 km/159.5 mi) east of Semisopochnoi and may represent short-lived explosions. No ash was detected in satellite imagery or reported by passing pilots.
Seismic tremor declined in early October and AVO lowered the color code and alert level at Semisopochnoi to YELLOW/Advisory on October 12, 2018, although discrete earthquake activity remained above background.
An eruptive event was detected in seismic data beginning at 4:47 UTC on October 26 (20:47 AKDT on October 25) and the Aviation Color Code was increased to Orange and the Volcano Alert Level to Watch as a result. This event was characterized by strong volcanic tremor that lasted for about 20 minutes, followed by about 40 minutes of weaker tremor pulses. The event also produced weak infrasound (pressure wave) that was detected by instruments on Adak Island. A thick meteorological cloud deck obscured observations below 10,000 ft asl and a very faint cloud was observed rising briefly above it in a satellite image. It is unclear whether this cloud was related to the eruptive activity or whether it contained any volcanic ash. Seismic tremor ended following this event and seismicity returned to low levels with occasional earthquake activity. Several small explosive events occurred after this explosion, although no ash clouds were detected in satellite imgaery from them. The satellite link that transmits seismic data from Semisopochnoi failed later on Thursday, November 1 but an infrasound array on Adak Island could still detect significant explosive emissions from Semisopochnoi (with a 13-minute delay) if atmospheric conditions permit.
On November 21, 2018, citing no eruptive activity since November 1, AVO lowered the aviation color code and volcano alert level to YELLOW/Advisory. No eruptive activity was observed in satellite imagery or regional infrasound throughout November and December.
On December 19, 2018, AVO lowered Semisopochnoi to UNASSIGNED.
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.