Earthquake activity has increased at Semisopochnoi in the Aleutian Islands. Heightened activity began in January and has progressed to the point where brief periods of seismic tremor have been observed. This may indicate movement of magma or volcanic gasses. While the current swarm has not yet surpassed the June – July 2014 swarm in amplitude or earthquake frequency, it is well on its way. Alaska Volcano Observatory activity page can be found here: http://www.avo.alaska.edu/activity/Semisopochnoi.php
Ashfall prediction for March 26 can be found here: http://www.avo.alaska.edu/activity/ash3d/ashfall.php?vid=ak248
Semisopochnoi Island is a basalt volcanic island some 2,100 km southwest from Anchorage. It sits approximately two thirds of the way along the Aleutian Island chain toward Kamchatka.
The island itself is some 20 km in diameter. It is topped with a caldera some 8 km in diameter. There are three stratovolcano cones and multiple parasitic cones currently building in the caldera. These include Sugarloaf, Anvil and Cerebus
Maximum height of the island above the Pacific is 1,221 m.
The island is uninhabited and there are no permanent settlements within 100 km. The closest town is Adak, some 204 km southeast.
Like most of the Aleutians, it is a home for birds.
Eruptive melt is supplied via the subduction of the Pacific Plate under the North American Plate. The Aleutian Trench is south of the island. It is part of the Rat Islands. The island sits on the end of a small submarine ridge in the Bering Sea called Bower’s Ridge. AVO does not know if the location of the ridge drives the production or composition of eruptive materials. http://www.avo.alaska.edu/volcanoes/volcinfo.php?volcname=Semisopochnoi
The island is a typical basaltic andesitic structure with multiple layers of lava and pyroclastic materials. The caldera forming eruption is thought to have taken place some 7950 BC. Total materials produced were on the order of 150 km3 leading to the creation of the caldera. It was mainly dacitic pumice and pyroclastic flows. The island appeared to be topped with glaciers at the time of the eruption. This eruption would have been a VEI7. http://www.bgs.ac.uk/vogripa/searchVOGRIPA.cfc?method=detail&id=1036
The most recent eruption was an explosive eruption out of Sugarloaf Peak, one of the stratovolcanoes growing inside the caldera. The eruption was a VEI2; took place in 1987 and lasted about six weeks starting in April 1987.
The most recent eruption before that was 1873. There are at least four earlier eruptions reported, though due to the remoteness of the island, actual details are few and far between.
Like all the Aleutians, Semisopochnoi has the ability to produce massive eruptions. It’s location under the air route across the North Pacific make any eruption an immediate concern for air traffic and resident wildlife, but little immediate concern for people. There is no prediction as yet about impending eruptions, though any earthquake swarm that shows occasional tremor is something to pay attention to.