Seismic Swarm S20180927.1: Analysis of Activity Near Adak, Alaska

Seismic swarm S20180927.1 occurred approximately 125 km west-southwest of Adak, Alaska, in the central Aleutian Islands. The sequence began at 05:55 UTC on 27 September 2018 and concluded at 15:14 UTC on 28 September 2018, spanning 33 hours and 19 minutes. During this period, 40 earthquakes were recorded, with magnitudes ranging from 1.4 to 4.8 and focal depths predominantly between 4 km and 18 km.

The largest event reached magnitude 4.8 at a depth of 10 km, occurring at the onset of the swarm. Subsequent activity included multiple events above magnitude 2.0, such as a magnitude 3.1 quake at 18 km depth and several magnitude 2.2–2.4 shocks clustered between 11 km and 14 km depth. Most events exhibited shallow crustal depths consistent with the tectonic setting, though one outlier registered at 52 km. The temporal distribution showed peak activity in the first 12 hours, followed by a gradual decline, characteristic of swarm behavior rather than a classic mainshock-aftershock sequence.

This swarm unfolded 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. The central Aleutians host frequent seismic swarms due to heterogeneous stress accumulation along the megathrust and associated crustal faults. Depths observed in S20180927.1 align with typical intraslab and upper-plate seismicity in this region. Historical records indicate elevated swarm frequency linked to the arc’s volcanic and tectonic framework, including nearby features such as the Adak Island volcanic complex.

Since 2000, eleven swarms have been documented in the broader Adak vicinity. These occurred in 2006 (two swarms), 2007 (one), 2008 (one), 2012 (two), 2013 (one), and 2017 (four). Such recurrent activity underscores the persistent strain release along this segment of the subduction interface, which has also produced great earthquakes, including the 1957 Mw 8.6 event and the 1965 Mw 8.7 Rat Islands earthquake.

Swarm S20180927.1 provides insight into episodic stress migration within the overriding plate and along the plate interface. The tight clustering of depths and the absence of a dominant mainshock suggest fluid involvement or aseismic slip triggering brittle failure, common in subduction-related swarms. No damage or tsunami effects were reported, consistent with the moderate magnitudes and offshore location.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center regional reports
SeismoSight internal swarm classification data