Seismic Swarm S20200511.1: Analysis of Activity Near Adak, Alaska

Seismic swarm S20200511.1 occurred approximately 107 km south-southeast of Adak, Alaska, in the Andreanof Islands region of the central Aleutian arc. The sequence began at 01:51 UTC on 11 May 2020 and concluded at 05:46 UTC on 12 May 2020, spanning 27 hours and 54 minutes. During this period, 32 earthquakes were recorded, with magnitudes ranging from 1.6 to 4.0 and focal depths predominantly between 6 km and 33 km.

The events clustered in two main phases. The initial phase on 11 May featured a magnitude 3.6 event at 01:51 UTC (10 km depth), followed by a magnitude 4.0 shock at 05:54 UTC (10 km depth). Subsequent activity included multiple events above magnitude 3.0 between 07:33 and 10:42 UTC, with the largest aftershock reaching magnitude 3.9 at 10:42 UTC. A secondary cluster occurred overnight, culminating in magnitude 3.8 and 3.9 events at 00:43 UTC and 01:54 UTC on 12 May, both at 10 km depth. Depths showed modest variation, with several events at 23–33 km suggesting possible involvement of the subducting slab interface.

This swarm represents the fourth such sequence in the region since 2000. Earlier swarms occurred in 2010 (three events) and 2017 (one event), indicating episodic clustered seismicity superimposed on the background tectonic regime.

The Adak area lies within the Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at rates of approximately 6–7 cm per year. This oblique subduction generates the Aleutian Trench and fuels both intermediate-depth intraslab earthquakes and shallow crustal events. The central Aleutians host frequent moderate-magnitude activity due to the presence of fracture zones and seamounts on the incoming plate, which can influence rupture segmentation. Historical records document major earthquakes in the Andreanof Islands, including the 1957 magnitude 8.6 event and the 1986 magnitude 8.0 shock, underscoring the region’s capacity for large-magnitude release.

Swarm activity in subduction settings often reflects fluid migration or stress transfer along pre-existing faults rather than mainshock-aftershock sequences. Depths recorded in swarm S20200511.1 align with both the megathrust interface and overlying crustal volumes, consistent with typical patterns observed along the arc.

References

USGS Earthquake Hazards Program – Aleutian Arc tectonics overview
Alaska Earthquake Center – Regional seismicity summaries
Incorporated Research Institutions for Seismology (IRIS) – Subduction zone characteristics