AVO reported that earthquake activity near Tanaga had decreased in both rate and magnitude. The rate of earthquakes had been about three events per day during the previous three weeks, much lower than the 150 events per day recorded during the peak of the swarm in mid-March. Recent satellite images did not indicate any deformation on the island and no other signs of volcanic unrest had been detected. On 18 July the Volcano Alert Level was lowered to Normal (the lowest level on a four-level scale) and the Aviation Color Code was lowered to Green (the lowest color on a four-color scale).

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)