Seismic Swarm S20070110.2: Analysis near Unalaska, Alaska

Earthquake swarm S20070110.2 occurred 76 km southwest of Unalaska in the Aleutian Islands, Alaska. The sequence began at 10:07 on 10 January 2007 and concluded at 15:59 on 13 January 2007, spanning 77 hours and 52 minutes. During this period, 108 earthquakes were recorded.

The Aleutian Islands lie along an active subduction zone where the Pacific Plate descends beneath the North American Plate at rates exceeding 6 cm per year. This tectonic setting produces frequent seismic swarms and volcanic activity across the arc. Unalaska sits near several stratovolcanoes, including Makushin, whose magma systems can trigger localized seismicity through fluid migration or pressure changes.

Analysis of the first 100 events reveals a magnitude range from 1.5 to 3.7, with the majority falling between 1.6 and 2.5. Depths clustered primarily between 10 km and 25 km, consistent with upper-crustal processes in the overriding plate. A small number of shallower events near 3 km suggest possible involvement of near-surface structures or hydrothermal activity. Temporal distribution showed peak activity in the initial hours, followed by a gradual decline, typical of swarm behavior rather than a mainshock-aftershock sequence.

Such swarms are common in the central Aleutians and often correlate with regional strain release or volcanic unrest. The 2007 event aligns with documented patterns of episodic seismicity in this highly active margin, where cumulative energy release occurs through numerous moderate events instead of isolated large shocks.

The Aleutian subduction zone has produced several historic great earthquakes, including the magnitude 8.6 event of 1957 and the magnitude 9.2 Great Alaska Earthquake of 1964. Ongoing monitoring by regional networks continues to track similar swarms, contributing to improved hazard assessment for nearby communities and infrastructure.

References
USGS Earthquake Hazards Program
Alaska Earthquake Center annual reports
NOAA National Centers for Environmental Information
SeismoSight internal swarm classification records