Seismic Swarm S20170508.1: Analysis of Activity Near Adak, Alaska

A notable earthquake swarm, designated S20170508.1, occurred approximately 127 km west-southwest of Adak, Alaska, in the central Aleutian Islands. The sequence began at 23:01 UTC on 7 May 2017 and concluded at 09:02 UTC on 11 May 2017, spanning 82 hours and producing 165 recorded events. This swarm reflects the intense tectonic activity characteristic of the region. Analysis of the first 100 events reveals a concentration of shallow seismicity. Magnitudes ranged from 2.5 to 4.5, with the largest event (M4.5) occurring at 22:22 on 8 May at a depth of 10 km. Depths varied between 1 km and 21 km, though the majority clustered near 10 km. Activity intensified on 8 May, featuring multiple events above M3.5 within a short window, including a cluster between 17:10 and 17:30 UTC. Subsequent days showed a gradual decline, with events becoming less frequent and generally smaller after 9 May. The temporal pattern indicates a classic swarm progression driven by fluid migration or stress transfer rather than a single mainshock-aftershock sequence. The Aleutian Islands lie along the convergent boundary where the Pacific Plate subducts beneath the North American Plate at rates exceeding 6 cm per year. This subduction generates the Aleutian megathrust and associated volcanic arc, producing frequent earthquakes at depths from shallow crustal levels to over 200 km. The central Aleutians, including the area near Adak, experience elevated seismicity due to the oblique convergence and complex slab geometry. Historical records document numerous swarms in this segment, consistent with episodic slip or magmatic processes. Since 2000, eight swarms have been identified in the immediate region, occurring in 2006 (two swarms), 2007, 2008, 2012 (two swarms), 2013, and 2017. These events underscore the persistent seismic hazard in the central Aleutians, where monitoring supports tsunami and volcanic risk assessment.

References

USGS Earthquake Hazards Program, Aleutian Arc tectonics overview. Alaska Earthquake Center regional seismicity reports.