Great Sitkin reported activity

Event Name : Great Sitkin 2016/7

Start:	June 30, 2017	Observed
Stop:	December 2017 ± 1 Months	Observed

Fumarolic or hydrothermal activity:

Description: From the AVO Volcano Activity Notice of November 22, 2017: Recent observations of a robust steam plume and a period of gradually increasing seismicity over several months indicate that Great Sitkin Volcano has become restless and is exhibiting behavior that is above background levels. AVO is thus raising the aviation color code and volcano alert level to YELLOW/ADVISORY.



Photographs of the volcano taken by local observers on Sunday, November 19 show a light-colored vapor plume rising about 300 m (1,000 ft) above the vent area and extending about 15-20 km (9 -12 mi) to the south. Nothing unusual was observed in seismic or infrasound data around the time the photographs were taken and nothing noteworthy has been observed in satellite data since the emissions were observed.



An increased number of small earthquakes was evident as early as late July 2016, and since then the level of seismic activity has fluctuated at low levels but has exhibited a gradual overall increase most notable since June 2017. Seismic activity to date has been characterized by earthquakes that are typically less than magnitude 1.0 and range in depth from near the summit of the volcano to 30 km below sea level. Most earthquakes are in one of two clusters, beneath the volcano's summit or just offshore the northwest coast of the island. The largest earthquake so far was a magnitude 2.8 on September 29, 2017.



Possible explosion signals were observed in seismic data on January 10 and July 21 of [2017] but no confirmed emissions were observed locally or detected in infrasound data or satellite imagery.

After two months of declining seismicity, AVO lowered Great Sitkin to Green/Normal on January 18, 2018.