Seismic Swarm PS20071002.1: Analysis of Activity Southeast of King Cove, Alaska

A seismic swarm designated PS20071002.1 occurred on 2 October 2007, approximately 78 km southeast of King Cove on the Alaska Peninsula. The sequence began at 18:00 UTC and concluded at 20:57 UTC, encompassing five earthquakes within a span of two hours and fifty-seven minutes. This event cluster reflects typical aftershock or swarm behavior in a tectonically active subduction environment. The earthquakes registered the following parameters in chronological order: at 18:00:06 UTC, a magnitude 6.3 event at 32 km depth; at 18:03:54 UTC, a magnitude 5.6 event at 47 km depth; at 20:07:46 UTC, a magnitude 2.7 event at 41 km depth; at 20:57:33 UTC, a magnitude 5.4 event at 30 km depth; and at 20:57:39 UTC, a magnitude 5.1 event at 42 km depth. Depths ranged between 30 and 47 km, consistent with intermediate-depth seismicity along the subducting slab. The Alaska Peninsula lies within the Aleutian subduction zone, where the Pacific Plate converges with and descends beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic interaction produces the Aleutian Trench and drives both megathrust earthquakes and volcanic activity along the arc. Historical records document repeated large-magnitude events in the region, including the 1938 magnitude 8.2 earthquake near Sand Point and the 1946 magnitude 8.6 Unimak Island event, both of which generated significant tsunamis. Ongoing monitoring by regional networks highlights persistent moderate seismicity due to slab dehydration and stress accumulation. In the broader timeframe since 2000, notable strong earthquakes have occurred nearby. These include the magnitude 7.2 Sand Point earthquake of 16 July 2023, centered 47 km from the swarm location, and the magnitude 7.3 Sand Point earthquake of 16 July 2025, located 70 km from the swarm center. Such events underscore the persistent seismic hazard along this segment of the plate boundary. The 2007 swarm likely represents a localized release of strain within the downgoing slab, distinct from the mainshock-aftershock sequences of larger subduction-zone ruptures. No significant surface rupture or volcanic unrest was associated with this activity, aligning with the intermediate depths observed.

References

United States Geological Survey Earthquake Catalog Alaska Earthquake Center Regional Reports SeismoSight Internal Swarm Classification Database