Seismic Swarm S20170509.1 Near Adak, Alaska: Analysis and Regional Context

A seismic swarm designated S20170509.1 occurred approximately 110 km south-southwest of Adak, Alaska, in the Andreanof Islands sector of the Aleutian chain. The sequence began at 15:36 on 8 May 2017 and concluded at 12:34 on 10 May 2017, spanning 44 hours and 57 minutes. During this interval, 34 earthquakes were recorded, with magnitudes ranging from 1.9 to 4.9 and focal depths predominantly between 10 km and 41 km.

The swarm initiated with a magnitude 4.9 event at 10 km depth, followed rapidly by additional events including magnitudes 4.3, 4.0, 4.6, and 3.8. Depths clustered near 10 km for many of the larger shocks, while smaller events extended to greater depths, reaching 41 km. Activity showed temporal clustering, with notable bursts on 8 May evening and 9 May morning, before tapering on 10 May. This pattern is characteristic of swarm sequences driven by fluid migration or stress transfer along fault networks rather than a single mainshock-aftershock cascade.

The location lies within the tectonically active Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at rates exceeding 6 cm per year. This setting produces frequent earthquakes and volcanic activity along the island arc. Adak itself sits near the Adak Caldera and other volcanic centers, underscoring the region's elevated seismic hazard.

Historical records indicate limited swarm occurrences in the immediate vicinity since 2000, with only one prior event in 2008. The 2017 swarm therefore represents a relatively infrequent phenomenon in the documented catalog for this specific offshore area.

Such sequences contribute to understanding stress accumulation and release along the subduction interface, aiding long-term seismic hazard assessment for the Aleutian Islands. Continued monitoring by regional networks remains essential given the potential for larger events in this high-strain environment.

References

USGS Earthquake Catalog
Alaska Earthquake Center reports
SeismoSight internal swarm classification data