Seismic Swarm S20150401.2: Earthquake Activity West of Adak, Alaska

Seismic swarm S20150401.2 occurred approximately 297 km west-southwest of Adak, Alaska, in the central Aleutian Islands. The sequence began at 16:38 UTC on 31 March 2015 and concluded at 12:53 UTC on 1 April 2015, spanning 20 hours and 15 minutes. During this period, 26 earthquakes were recorded, with magnitudes ranging from 1.7 to 4.1 and focal depths between 7 km and 30 km.

The swarm initiated with two magnitude-2.2 events at depths of 20 km and 18 km. Activity intensified several hours later, producing four events of magnitude 3.9–4.1 between 18:19 and 18:37 UTC on 31 March. Subsequent events remained predominantly below magnitude 3.0, although additional magnitude-3.9 and 3.8 shocks occurred at 22:53 UTC and 23:39 UTC. Depths showed modest variation, clustering most frequently between 13 km and 25 km.

The Aleutian Islands occupy a tectonically active subduction zone where the Pacific Plate converges with and descends beneath the North American Plate at rates exceeding 6 cm per year. This setting generates frequent intermediate-depth seismicity and occasional volcanic activity along the arc. Adak and the surrounding Andreanof Islands lie within a region characterized by complex faulting and repeated moderate-magnitude earthquake sequences.

Historical records indicate that ten earthquake swarms have been identified in the same general area since 1 January 2000. These occurred in 2003 (two swarms), 2005 (two), 2006 (three), 2007 (one), 2012 (one), and 2014 (one). Swarm sequences in this segment of the arc typically consist of low-to-moderate magnitude events distributed over hours to days, consistent with the pattern observed in S20150401.2.

The 2015 swarm displayed classic swarm characteristics: rapid onset, clustered timing, absence of a single dominant mainshock, and relatively shallow focal depths. Such sequences are commonly attributed to fluid migration or localized stress adjustments within the overriding plate or along the plate interface.

References

• USGS Earthquake Catalog (earthquake.usgs.gov)
• Alaska Earthquake Center, University of Alaska Fairbanks
• SeismoSight internal swarm classification records