Seismic Swarm S20131113.2 Near Adak, Alaska: Geological Context and Event Analysis

Seismic swarm S20131113.2 was recorded 152 km west-southwest of Adak, Alaska, in the Andreanof Islands region of the central Aleutian arc. The sequence began at 04:00 UTC on 13 November 2013 and concluded at 00:58 UTC on 17 November 2013, spanning 92 hours and 57 minutes. During this period, 78 earthquakes were registered, with magnitudes ranging from 1.7 to 5.3 and focal depths primarily between 0 and 31 km.

The swarm exhibited a typical pattern of clustered, moderate-magnitude events without a single dominant mainshock. The largest event reached magnitude 5.3 at a depth of 25 km on 15 November at 03:35 UTC. Other notable shocks included magnitudes 4.3, 4.0, 3.8, and multiple events above 3.5, occurring predominantly in the first 48 hours. Activity then declined gradually, with smaller events persisting through 16 November before tapering off. Depths clustered around 5–15 km for the majority of events, consistent with shallow crustal processes in the overriding plate.

The location lies within the Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at approximately 6–8 cm per year. This tectonic setting produces frequent seismicity, including both interplate thrust earthquakes and intraplate activity within the subducting slab and the overlying volcanic arc. The central Aleutians host active volcanoes such as Adak and Great Sitkin, whose magma systems can influence local stress fields and contribute to swarm-type seismicity through fluid migration or pressure changes.

Historically, the region has experienced significant earthquakes, including the magnitude 8.6 Andreanof Islands event of 1957 and the magnitude 7.9 quake of 1996. Since 2000, four earthquake swarms have been documented in the immediate area, with earlier episodes in 2006 (two swarms) and 2007 (two swarms). These recurrent swarms highlight the persistent stress accumulation and release characteristic of the subduction interface and adjacent fault systems.

Analysis of swarm S20131113.2 indicates a diffuse spatial distribution consistent with triggered activity along secondary faults or within the volcanic-arc crust rather than a single through-going rupture. The absence of aftershock decay typical of mainshock–aftershock sequences further supports classification as a swarm. Such events provide valuable data for refining seismic hazard models in the Aleutian arc, where remote monitoring remains essential due to the archipelago’s limited infrastructure.

References
SeismoSight internal swarm catalog (S20131113.2 parameters and statistics).
USGS Earthquake Catalog (regional tectonic framework and historical events).
Alaska Earthquake Center (Aleutian arc seismicity overview).