Seismic Swarm S20101104.2: Analysis of Activity Near Adak, Alaska

A significant seismic swarm, internally classified as S20101104.2, occurred approximately 100 km south-southeast of Adak in the Aleutian Islands of Alaska. The sequence began at 23:31 UTC on 3 November 2010 and concluded at 08:48 UTC on 8 November 2010, spanning 105 hours and 17 minutes. During this period, 177 earthquakes were recorded, with detailed parameters available for the initial 100 events.

The Aleutian Islands lie along the convergent boundary where the Pacific Plate subducts beneath the North American Plate at rates exceeding 6 cm per year. This tectonic setting produces frequent seismicity, including both shallow crustal events and deeper Wadati-Benioff zone activity. Depths in the swarm ranged from near-surface to approximately 49 km, consistent with the regional subduction geometry. Magnitudes varied from 2.0 to 4.6, with several events reaching or exceeding 4.5. The first 100 events showed clustering in both time and depth, with many occurring between 15 and 28 km, indicating activity within the overriding plate and along the plate interface.

Notable larger events included a magnitude 4.3 at 01:11 on 4 November at 16 km depth, followed closely by a magnitude 4.6 at 42 km. Additional magnitude 4.6 earthquakes occurred at 06:16 and 06:23 on 4 November, and again at 08:01, each at depths between 16 and 26 km. These larger shocks were interspersed with numerous smaller events, many registering between 2.0 and 3.5. Temporal patterns revealed bursts of activity, particularly in the early hours of 4 November and again around midday, suggesting episodic stress release along the subduction interface.

The Adak region has a well-documented history of intense seismic activity due to its position in the central Aleutians. Major historical earthquakes include the 1957 magnitude 8.6 Andreanof Islands event and the 1965 magnitude 8.7 Rat Islands earthquake, both of which generated significant tsunamis. Ongoing monitoring by the Alaska Earthquake Center and the USGS confirms that the area experiences hundreds of earthquakes annually, with swarms commonly associated with fluid migration or aseismic slip transients in the subduction zone.

Analysis of the first 100 events indicates a typical swarm signature: high event rates without a single dominant mainshock-aftershock sequence. Depths predominantly clustered around 25 km, with shallower outliers possibly reflecting activity in the forearc crust. Magnitudes showed no clear systematic migration, though the largest events distributed throughout the sequence rather than at the onset. Such patterns align with known behavior in the Aleutian subduction zone, where swarms often occur near the downdip limit of the seismogenic zone.

This swarm contributes to the broader understanding of seismic hazards in the central Aleutians. Continued instrumentation and real-time monitoring remain essential for assessing potential escalation to larger events. Updated regional models from the USGS incorporate these data to refine estimates of recurrence intervals for great earthquakes along the arc.

References

• USGS Earthquake Catalog (earthquake.usgs.gov)
• Alaska Earthquake Center, University of Alaska Fairbanks
• Tectonic summaries of the Aleutian subduction zone, USGS Professional Papers