Seismic Swarm S20240419.2: Analysis of Activity Near Adak, Alaska

A seismic swarm designated S20240419.2 was recorded 57 km west-southwest of Adak, Alaska, in the Aleutian Islands. The sequence began at 16:36 UTC on 18 April 2024 and concluded at 00:54 UTC on 1 May 2024, spanning 296 hours and 17 minutes. During this period, 252 earthquakes were registered.

The Aleutian Islands occupy a tectonically active segment of the Pacific Ring of Fire. Here, the Pacific Plate subducts beneath the North American Plate along the Aleutian Trench, generating frequent seismic events and volcanic activity. This subduction process produces earthquakes at varying depths, with many occurring in the shallow to intermediate range consistent with the observed swarm depths of 0–16 km. Adak lies within this dynamic zone, where historical seismicity reflects ongoing plate convergence at rates of approximately 6–8 cm per year.

Analysis of the first 100 events reveals a predominance of low-magnitude earthquakes. The initial event measured magnitude 2.0 at 11 km depth. Subsequent activity included multiple events below magnitude 2.0, interspersed with larger shocks such as a magnitude 4.1 at 10 km depth on 19 April at 12:53 UTC. Other notable magnitudes reached 3.8 on two occasions later that day. Depths clustered primarily between 6 and 13 km, with occasional shallower or deeper outliers. The sequence exhibited typical swarm characteristics: a rapid onset of small events building to moderate peaks without a single dominant mainshock.

This swarm aligns with the region's established pattern of clustered seismicity. Since 1 January 2000, seven swarms have occurred in the area, with prior episodes recorded in 2008 (two swarms), 2017, 2020, 2021, 2023, and an earlier 2024 event. Such recurrent activity underscores the persistent stress accumulation and release along the subduction interface near Adak.

The data indicate no immediate escalation to higher magnitudes within the initial events, though continued monitoring remains essential given the area's volcanic and seismic setting. Overall, the swarm provides further evidence of the Aleutian subduction zone's ongoing geodynamic behavior.

References

SeismoSight internal swarm classification records
USGS Earthquake Catalog for Aleutian region tectonics