Seismic Swarm S20120927.1 Near Adak, Alaska: Geological Context and Event Analysis

The seismic swarm designated S20120927.1 occurred approximately 111 km west-southwest of Adak, Alaska, in the central Aleutian Islands. It began at 23:46 UTC on 26 September 2012 and concluded at 09:31 UTC on 7 October 2012, spanning 249 hours and 45 minutes. During this period, 594 earthquakes were recorded. This activity aligns with the region's position along the Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at rates of about 6–7 cm per year.

The Aleutian Islands form a volcanic arc resulting from oblique subduction. The area experiences frequent seismicity due to thrust faulting along the plate interface, as well as crustal extension and strike-slip motion. Adak Island itself lies near the Adak Canyon and is influenced by the Andreanof segment of the subduction zone, which has produced several magnitude 7+ events historically. Depths of events in this swarm predominantly ranged from 0 to 41 km, consistent with both shallow crustal and intermediate-depth activity typical of the Wadati-Benioff zone.

Analysis of the first 100 events reveals magnitudes between 1.9 and 4.6, with the largest being a 4.6 quake at 23 km depth on 27 September 2012 at 03:28:45 UTC. Depths clustered mostly between 10 and 25 km, though outliers reached 41 km. Early activity showed several events above magnitude 3.0 within the first hour, followed by a gradual decline in both frequency and maximum magnitude. Notable events included a 4.1 quake at 9 km depth at 02:49:10 UTC and multiple 3.0–3.4 events in the initial 24 hours. Depths showed no strong correlation with magnitude in this subset, indicating distributed fault slip rather than a single plane.

Historical records indicate three swarms in the region since 1 January 2000. Prior episodes occurred in 2006 (two swarms) and 2007 (one swarm). Such clustered activity is not uncommon in the Aleutians, where aftershock sequences and swarm-like behavior often accompany stress transfer along the megathrust.

This swarm underscores the persistent seismic hazard in the central Aleutians, where monitoring supports tsunami and earthquake preparedness for nearby communities and infrastructure.

References

SeismoSight internal swarm classification data (S20120927.1 parameters and event catalog).
USGS Earthquake Hazards Program: Aleutian Islands tectonics and historical seismicity summaries.