Earthquake Swarm S20070816.1 Near Adak, Alaska: Seismological Analysis

The Aleutian Islands form a classic intra-oceanic subduction zone where the Pacific Plate converges with the North American Plate at approximately 7–8 cm per year. This tectonic setting produces frequent moderate-to-large earthquakes and occasional earthquake swarms. The study region, centered 173 km SSW of Adak, lies along the central Aleutian arc segment characterized by a well-developed Wadati-Benioff zone and active volcanism.

Swarm S20070816.1 began at 20:22 UTC on 15 August 2007 and concluded at 21:43 UTC on 20 August 2007, spanning 121 hours and 20 minutes. During this interval, 191 earthquakes were recorded. The sequence opened with a magnitude 6.5 event at 9 km depth, followed within minutes by a magnitude 3.5 shock at 29 km. Subsequent activity included numerous events between magnitude 2.0 and 3.9, with focal depths predominantly between 4 km and 25 km. Two larger events punctuated the swarm: a magnitude 5.8 earthquake at 14 km depth on 16 August at 04:03 UTC and a magnitude 4.6 shock at 19 km depth later the same day.

Analysis of the first 100 events reveals a rapid initial decay in event rate after the magnitude 6.5 mainshock, consistent with aftershock behavior, yet the spatial clustering and lack of a single dominant rupture plane support swarm classification. Depths remained shallow to mid-crustal, suggesting brittle failure within the overriding plate and upper portion of the subducting slab. Magnitudes decreased steadily, with only four events exceeding magnitude 3.5 in the initial 100-event window.

The Aleutian arc has a well-documented history of great earthquakes, including the 1957 M8.6 Andreanof Islands event and the 1965 M8.7 Rat Islands earthquake, both located west of the 2007 swarm. Modern seismic monitoring by the Alaska Earthquake Center and USGS confirms that swarms of this duration and productivity occur several times per decade in the central Aleutians, often linked to fluid migration or stress transfer along the plate interface.

No significant damage or tsunami was reported from this swarm, reflecting both its offshore location and moderate energy release. Continued monitoring of the region remains essential given the arc’s capacity for much larger events.

References

USGS Earthquake Catalog
Alaska Earthquake Center Regional Seismicity Reports
Plafker, G., & Berg, H. C. (1994). The Geology of Alaska. Geological Society of America.