Seismic Swarm S20091015.1: Insights into the 2009 Unalaska Sequence

The seismic swarm designated S20091015.1 was recorded 128 km south of Unalaska, Alaska, in the central Aleutian Islands. It began at 00:13 UTC on 15 October 2009 and concluded at 11:32 UTC on 19 October 2009, spanning 107 hours and 19 minutes. During this interval, 82 earthquakes were detected, with magnitudes ranging from 1.6 to 5.5 and focal depths primarily between 1 km and 56 km.

This sequence exemplifies a classic earthquake swarm, characterized by a rapid onset of events without a single dominant mainshock followed by aftershocks. The largest event, magnitude 5.5 at 20 km depth, occurred at the swarm's initiation. Subsequent notable shocks included a magnitude 4.9 at 10 km depth roughly three hours later and several magnitude 3+ events distributed throughout the first two days. Depths clustered around 20–25 km for many events, consistent with activity near the plate interface, while shallower events (under 10 km) and deeper ones (over 40 km) indicate a vertically distributed rupture zone.

The Unalaska region lies within the Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at approximately 6–7 cm per year. This tectonic setting produces the Aleutian volcanic arc and drives frequent seismicity. The swarm's location aligns with the megathrust boundary, where stress accumulation and fluid migration can trigger clustered seismicity without a clear foreshock-mainshock-aftershock progression.

Historically, the Aleutian Islands have hosted some of the largest earthquakes on record, including the 1957 M8.6 Andreanof Islands event and the 1965 M8.7 Rat Islands earthquake. These great subduction events underscore the region's capacity for both megathrust ruptures and smaller swarm activity. Since 2000, only two swarms have been identified in the immediate area, with S20091015.1 representing the first documented instance.

Analysis of event timing reveals peak activity within the initial 24 hours, followed by a gradual decline. Magnitudes generally decreased after the opening events, though intermittent magnitude 3+ shocks persisted until 17 October. This temporal pattern suggests episodic stress release along a segmented fault patch, possibly influenced by pore-pressure changes at depth.

Ongoing monitoring by regional networks continues to refine understanding of swarm mechanics in subduction environments. Such sequences provide valuable data for assessing background seismicity rates and potential precursory signals ahead of larger events in the Aleutian arc.

References

USGS Earthquake Catalog
Alaska Earthquake Center, University of Alaska Fairbanks
SeismoSight internal swarm classification records