The Alaska Volcano Observatory (AVO) has detected an increase in seismic activity at Great Sitkin Volcano in the central Aleutian Islands, possibly starting as early as 30 July 2016.
The activity is characterized by small earthquakes that are typically less than magnitude 1.0, and range in depth from near the summit of the volcano to 5 km below sea level.
Since July 2016, AVO has located 462 earthquakes in this region, as opposed to 44 earthquakes during the same time period a year prior.
In addition to shallow quakes, we have also seen a slight increase in seismicity at depths of 18 to 32 km below sea level that is consistent with the involvement of fluids in the deeper earthquakes. Seismic signals on 10 January 21:05 AKST (11 January 06:05 UTC) and 21 July 16:17 AKDT (22 July 00:17 UTC) of this year may represent small explosions near the summit of Great Sitkin.
While these signals have been observed seismically, the occurrence of explosive activity has not been detected in either infrasound data or satellite imagery.
AVO has not received any reports of explosive activity from passing aircraft or residents of Adak for either the January 10 or July 22 events.
The activity that we have observed at Great Sitkin over the past months is consistent with magma intrusion beneath the volcano.
Such an intrusion will release gas, which can create deep seismic signals as well as increase stress at shallower depths, leading to an increased number of shallow earthquakes.
If gases build up near the surface, they may be released suddenly in small explosions.
The character of observed seismicity indicates such explosions likely took place at Great Sitkin in January and July.
These observations suggest that the likelihood of a volcanic eruption has increased at Great Sitkin. Prior to a significant eruption, we expect an even greater increase in seismicity as magma rises to shallower levels in the crust.
If Great Sitkin were to erupt, the primary hazard would be airborne ash, and ash fall on communities (primarily Adak, 26 miles to the west) or marine vessels in the region.
Great Sitkin Volcano is a basaltic andesite volcano that occupies most of the northern half of Great Sitkin Island, a member of the Andreanof Islands group in the central Aleutian Islands.
It is located 43 km (26 miles) east of the community of Adak.
The volcano is a composite structure consisting of an older decapitated volcano and a younger parasitic cone with a 2-3 km diameter summit crater.
A steep-sided lava dome, emplaced during an eruption in 1974, occupies the center of the crater. Great Sitkin erupted at least three times in the 20th century, most recently in 1974.
That eruption produced a lava dome and at least one ash cloud that likely exceeded an altitude of 25,000 ft above sea level.
A poorly documented eruption occurred in 1945, also producing a lava dome that was partially destroyed in the 1974 eruption.
Within the past 280 years a large explosive eruption produced pyroclastic flows that partially filled the Glacier Creek valley on the southwest flank.