Seismic Swarm S20180815.1 Southwest of Adak, Alaska: Geological Context and Event Analysis

The region 110 km southwest of Adak, Alaska, 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 one of the most seismically active areas on Earth, characterized by frequent earthquakes, volcanic arcs, and occasional seismic swarms. The Aleutian Islands form part of the circum-Pacific Ring of Fire, with Adak situated near the Andreanof Islands segment, known for both moderate-to-large thrust earthquakes and clusters of smaller events at intermediate depths.

Seismic swarms in this region reflect stress adjustments along the plate interface and within the overriding crust, often without a single dominant mainshock. Historical records since 2000 indicate only four prior swarms in the immediate vicinity, occurring in 2005, 2006, 2007, and 2017. These episodes underscore the episodic nature of clustered seismicity amid the broader background of subduction-driven activity.

Swarm S20180815.1 initiated at 12:17 on 15 August 2018 and concluded at 13:39 on 19 August 2018, spanning 97 hours and 21 minutes. During this interval, 110 earthquakes were recorded. Analysis of the first 100 events reveals magnitudes ranging from 1.2 to 4.5, with the majority falling between 1.4 and 2.8. Depths predominantly clustered between 20 km and 45 km, consistent with activity along the subduction interface and within the Wadati-Benioff zone. Notable events included a magnitude 4.3 earthquake at 35 km depth shortly after onset, a magnitude 4.5 at 39 km depth on 16 August, and several magnitude 3.0–3.8 shocks distributed through the first two days. Temporal distribution showed peak activity in the initial 24–36 hours, followed by a gradual decline, with events continuing at lower rates until the swarm's termination.

Depth variations suggest involvement of both the megathrust and intraslab faulting, typical for Aleutian seismicity where events at 30–50 km often indicate dehydration embrittlement or slab bending. Magnitudes remained modest overall, with only four events exceeding magnitude 3.5, aligning with swarm characteristics rather than aftershock sequences of larger mainshocks.

This episode contributes to understanding swarm recurrence in the Andreanof region, where similar low-to-moderate clustered activity has been documented in prior years. Continued monitoring supports regional hazard assessment given the area's history of generating great earthquakes, such as the 1957 magnitude 8.6 Andreanof Islands event.

References

United States Geological Survey Earthquake Catalog (USGS).
Alaska Earthquake Center, University of Alaska Fairbanks.
SeismoSight internal swarm classification records.