Seismic Swarm PS20170509.1: Analysis of Activity South of Adak, Alaska

The seismic swarm designated PS20170509.1 occurred approximately 105 km south of Adak, Alaska, in the Andreanof Islands region of the Aleutian arc. This area lies within the tectonically active Pacific Ring of Fire, where the Pacific Plate subducts beneath the North American Plate along the Aleutian Trench. The subduction process generates frequent earthquakes and volcanic activity, with the arc exhibiting a history of both moderate swarms and major events exceeding magnitude 8.

The swarm commenced at 15:31 UTC on 8 May 2017 and concluded at 09:01 UTC on 9 May 2017, spanning 17 hours and 30 minutes. During this interval, five earthquakes were recorded, with magnitudes ranging from 5.2 to 5.9 and focal depths between 10 km and 19 km. These events clustered tightly in time and space, consistent with swarm behavior driven by fluid migration or stress transfer along the subduction interface rather than a single mainshock-aftershock sequence.

The sequence began with a magnitude 5.7 event at 15:31:26 on 8 May at 17 km depth, followed two minutes later by a magnitude 5.2 shock at 10 km depth. A magnitude 5.9 event at 15:47:29, also at 17 km depth, represented the largest in the swarm. Activity then paused until the following day, when a magnitude 5.5 earthquake occurred at 08:59:03 at 16 km depth, succeeded two minutes later by a magnitude 5.3 event at 19 km depth. Depths remained within the upper portion of the subducting slab, typical for intermediate-depth seismicity in this segment of the arc.

Since 1 January 2000, twenty seismic swarms have been documented in the same locale. Earlier episodes occurred in 2005 (one swarm), 2006 (two), 2007 (three), 2008 (three), 2010 (five), 2012 (two), 2013 (two), 2015 (one), and 2017 (one, the present case). This recurrence underscores persistent strain accumulation along the plate boundary.

The Aleutian subduction zone has produced several of the largest earthquakes in U.S. history, including the 1957 magnitude 8.6 Andreanof Islands event and the 1965 magnitude 8.7 Rat Islands earthquake. Modern instrumentation reveals that swarm activity often precedes or accompanies periods of elevated background seismicity, although individual swarms rarely trigger great earthquakes. Ongoing monitoring by the Alaska Earthquake Center and the U.S. Geological Survey continues to track deformation and microseismicity in the region.

• U.S. Geological Survey Earthquake Catalog
• Alaska Earthquake Center regional reports
• SeismoSight internal swarm classification records