At 2215 on 7 March AVO raised the Aviation Color Code for Tanaga to Yellow and the Volcano Alert Level to Advisory due to increased seismicity. Earlier that afternoon, starting at about 1330, seismicity began to increase and by around 2045 earthquakes were occurring at a rate of 2-3 per minute. The events were located at shallow depths and the largest events were M2-3.

Geological Summary. Tanaga volcano, the second largest volcanic center of the central Aleutians, is the central and highest of three youthful stratovolcanoes oriented along a roughly E-W line at the NW tip of Tanaga Island. Ridges to the east and south represent the rim of an arcuate caldera formed by collapse of an edifice during the Pleistocene. Most Holocene eruptions originated from Tanaga volcano itself, which consists of two large cones, the western of which is the highest, constructed within a caldera whose 400-m-high rim is prominent to the SE. At the westernmost end of the complex is conical Sajaka, a double cone that may be the youngest of the three volcanoes. Sajaka One volcano collapsed during the late Holocene, producing a debris avalanche that swept into the sea, after which the Sajaka Two cone was constructed within the collapse scarp.

Source: US Geological Survey Alaska Volcano Observatory (AVO)