Seismic Swarm S20230613.1: Analysis of Activity Near Adak, Alaska

A seismic swarm designated S20230613.1 occurred approximately 135 km west-southwest of Adak, Alaska, in the central Aleutian Islands. The sequence began at 16:16 on 13 June 2023 and concluded at 23:34 on 14 June 2023, spanning 31 hours and 18 minutes. During this interval, 40 earthquakes were recorded.

The largest event reached magnitude 5.3 at a depth of 19 km, occurring at the swarm's onset. Subsequent activity included multiple events of magnitude 3.3 at depths between 19 km and 23 km, alongside numerous smaller shocks ranging from magnitude 1.0 to 2.8. Depths clustered primarily between 9 km and 24 km, consistent with crustal processes in the region. The temporal distribution showed highest frequency in the first several hours, followed by a gradual decline, with isolated events persisting into the following day.

This swarm unfolded within the tectonically active 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 seismicity due to this oblique subduction, which generates both megathrust earthquakes and intermediate-depth events along the Wadati-Benioff zone. Adak Island lies near the Andreanof Islands segment, a portion of the arc characterized by complex faulting and volcanic arcs that have produced notable historical earthquakes, including the magnitude 8.6 event of 1957.

Seismic swarms in this setting often reflect fluid migration, stress transfer along the plate interface, or minor slip on subsidiary faults rather than a single mainshock-aftershock sequence. The 2023 swarm aligns with patterns observed in prior episodes, where clustered shallow-to-intermediate depth activity occurs without escalation to larger mainshocks.

Historical records indicate 13 swarms in the Adak region since 1 January 2000. These occurred in the following years with associated event counts: 2006 (2), 2007 (1), 2012 (1), 2013 (2), 2017 (4), 2018 (1), 2019 (1), and 2020 (1). Such recurrent swarms underscore the persistent strain accumulation and release along this portion of the subduction interface.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center (earthquake.alaska.edu)
SeismoSight internal swarm classification S20230613.1