Seismic Swarm S20080502.3: Analysis of Activity West of Adak, Alaska

An earthquake swarm designated S20080502.3 was recorded 84 km west of Adak, Alaska, in the Andreanof Islands region of the Aleutian arc. The swarm began at 01:46 on 2 May 2008 and concluded at 07:14 on 12 May 2008, spanning 245 hours and 27 minutes. During this interval, 184 earthquakes were registered. This event represents the sole swarm documented in the area since 1 January 2000, with no subsequent swarms recorded in the provided historical statistics.

The first 100 events of the swarm occurred primarily between 2 and 5 May 2008. Magnitudes ranged from 0.7 to 3.7, with the majority falling between 1.0 and 2.5. The largest event reached magnitude 3.7 at a depth of 7 km early in the sequence. Depths remained shallow throughout, varying from 0 km to 11 km, indicating activity within the upper crustal layers. Temporal distribution showed higher frequency in the initial 48 hours, with events clustering at intervals of minutes to hours before gradually declining. Notable clusters included multiple magnitude 1.9–2.3 events on 2 May and a series of smaller events (magnitudes 0.9–2.1) on 3–4 May. No events exceeded magnitude 4.0 in this subset, consistent with typical swarm behavior rather than a mainshock-aftershock sequence.

The location lies 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. This setting produces frequent seismicity, including both shallow crustal events and deeper Wadati-Benioff zone activity. The Andreanof Islands segment has a well-documented history of large earthquakes, such as the 1957 magnitude 8.6 event and the 1986 magnitude 8.0 earthquake, both of which generated significant ground shaking and tsunamis. Volcanic features in the arc, including nearby islands with Holocene activity, further underscore the region's dynamic geology driven by plate boundary processes.

Analysis of swarm characteristics suggests fluid migration or stress redistribution along pre-existing faults as potential triggering mechanisms, common in subduction-related swarms. The shallow depths and low magnitudes align with observations from other Aleutian seismic episodes, where swarms often precede or accompany volcanic unrest or afterslip. Updated regional monitoring by the Alaska Earthquake Center continues to track similar activity patterns in the arc, emphasizing the persistent seismic hazard.

References

• Alaska Earthquake Center, University of Alaska Fairbanks (seismic catalog data).
• USGS Earthquake Hazards Program (tectonic framework of the Aleutian arc).