Seismic Swarm S20150905.1: Analysis of Activity South of Atka, Alaska

Seismic swarm S20150905.1 occurred 92 km south of Atka, Alaska, in the Aleutian Islands. The sequence began at 05:48 on 5 September 2015 and concluded at 17:05 on 7 September 2015, spanning 59 hours and 17 minutes. During this interval, 103 earthquakes were recorded.

The first 100 events provide a detailed view of swarm evolution. Magnitudes ranged from 1.3 to 5.5, with the largest event (M5.5) occurring at 07:00:01 on 5 September at a depth of 20 km. Several other notable events exceeded M4.0, including an M4.7 at 07:19:58 (15 km depth), an M5.0 at 07:08:47 (20 km), and multiple M4.2–M4.6 shocks distributed through the first day. Depths clustered primarily between 10 km and 30 km, though shallower events (3–8 km) and deeper ones (up to 47 km) also appeared. Activity was most intense during the initial 12 hours, with frequent events above M3.0, followed by a gradual decline in both frequency and magnitude.

The Aleutian Islands lie along the convergent boundary where the Pacific Plate subducts beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces the region’s high seismicity and active volcanism. Atka itself sits on Atka Island within a chain of stratovolcanoes and calderas, including Korovin and Kliuchef, that have erupted multiple times in the Holocene. Historical records document significant earthquakes and swarms associated with both subduction-related thrust faulting and volcanic processes.

Since 1 January 2000, eight swarms have been identified in the broader area. Earlier episodes occurred in 2008 (two swarms) and 2013 (five swarms), with the present 2015 event representing the sole swarm recorded that year. These recurrent swarms underscore the persistent tectonic stress accumulation along the Aleutian megathrust and associated crustal faults.

Overall, swarm S20150905.1 exemplifies typical subduction-zone microseismicity, characterized by a rapid onset, a wide magnitude range, and a relatively short duration. Continued monitoring of this segment of the Aleutian arc remains essential for understanding stress transfer and potential volcanic interaction.