Seismic Swarm S20170501.2: Analysis of Activity West of Adak, Alaska

The Aleutian Islands form a volcanic arc resulting from subduction of the Pacific Plate beneath the North American Plate along the Aleutian Trench. This tectonic setting produces frequent seismicity, with Adak situated near the Andreanof Islands segment where convergence rates average 6–7 cm per year. The region has recorded numerous moderate-to-large earthquakes historically, including events exceeding magnitude 7 that have generated tsunamis. SeismoSight internal records document Swarm S20170501.2 beginning at 21:00 UTC on 30 April 2017 and concluding at 07:05 UTC on 24 May 2017. The sequence lasted 562 hours and 4 minutes, registering 1,627 earthquakes at a location 130 km west-southwest of Adak. Depths for the initial 100 events ranged primarily between 1 km and 21 km, consistent with shallow crustal and upper-plate seismicity above the subduction interface. Magnitudes in the first 100 events remained modest, with the majority between 1.5 and 3.0. The strongest signals reached 4.6 on two occasions (18:45 and 18:57 UTC on 3 May), accompanied by additional events of 3.7, 3.6, and 3.5. These larger shocks clustered within a 12-hour window on 3 May, after which activity gradually declined. Depths showed no systematic migration, suggesting distributed failure on multiple small faults rather than a single propagating rupture. Six prior swarms have occurred in the same region since 1 January 2000. These episodes took place in 2006 (two swarms), 2007 (one swarm), 2008 (two swarms), and 2013 (one swarm). The 2017 sequence fits the established pattern of episodic, low-to-moderate magnitude unrest typical of the Andreanof segment. Collectively, the data illustrate background swarm behavior driven by ongoing plate convergence and fluid migration within the overriding plate. Continued monitoring remains essential given the proximity to populated Aleutian communities and critical infrastructure.

References

USGS Earthquake Catalog (historical Aleutian seismicity) Alaska Earthquake Center regional reports (tectonic setting)