Seismic Swarm S20121018.1: Analysis of Activity Near Adak, Alaska

Seismic swarm S20121018.1 occurred approximately 109 km west-southwest of Adak, Alaska, in the Aleutian Islands. The sequence began at 07:27 UTC on 17 October 2012 and concluded at 15:56 UTC on 20 October 2012, spanning 80 hours and 28 minutes. During this period, 60 earthquakes were recorded.

The events exhibited magnitudes between 0.9 and 3.1, with focal depths ranging from 3 km to 24 km. Activity remained moderate throughout, featuring several events of magnitude 2.5 or greater, including peaks of 3.1 on 19 October at 01:09 UTC and again at 02:31 UTC, both at 12 km depth. Additional notable shocks reached 3.1 at 10:16 UTC on 20 October at 18 km depth. Depths clustered predominantly around 12–13 km, though shallower events near 3–5 km and deeper ones up to 24 km occurred intermittently. The temporal distribution showed higher frequency on 17–18 October, tapering gradually by 20 October.

This swarm represents clustered seismicity without a dominant mainshock, consistent with fluid migration or stress redistribution along faults in the subduction interface. Magnitudes stayed below levels that would typically trigger significant aftershock sequences, and the compact spatial footprint aligns with localized strain release.

Since 2000, four such swarms have been documented in the region. Earlier episodes occurred in 2006 (two swarms) and 2007 (one swarm), indicating recurrent swarm behavior in this segment of the arc.

The Adak area lies within the Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces the Aleutian volcanic arc and drives frequent earthquakes across a range of depths. Shallow crustal events often relate to upper-plate faulting, while intermediate-depth activity reflects slab dehydration and phase changes. Historical records document major megathrust earthquakes in the Aleutians, including the 1957 Mw 8.6 event near Adak, underscoring the zone’s capacity for large-magnitude releases alongside smaller swarm-type sequences.

Ongoing monitoring by regional seismic networks continues to track such activity, contributing to improved understanding of subduction dynamics and hazard assessment in the Aleutian Islands.

References
SeismoSight internal classification data, swarm S20121018.1
USGS Earthquake Hazards Program regional tectonic summaries
Alaska Earthquake Center historical seismicity reports