Seismic Swarm S20070802.1: Analysis of Activity Near Adak, Alaska

The seismic swarm designated S20070802.1 occurred approximately 228 km west-southwest of Adak, Alaska, in the central Aleutian Islands. It began at 01:41 UTC on 2 August 2007 and concluded at 16:11 UTC on 6 August 2007, spanning 110 hours and 29 minutes. During this interval, 147 earthquakes were recorded. The first 100 events exhibited magnitudes ranging from 1.7 to 5.3, with the majority occurring at shallow depths between 4 km and 21 km. The largest event reached magnitude 5.3 at a depth of 14 km early in the sequence, followed by several events above magnitude 4.0, including magnitudes 4.6, 4.1, and two of magnitude 4.0. Depths remained predominantly crustal, consistent with activity along the subduction interface.

This swarm took place within the Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate. Convergence rates in this segment average 7–8 cm per year, driving frequent seismicity and arc volcanism. The central Aleutians host a well-documented history of both great earthquakes and episodic swarms, often linked to stress transfer along the plate boundary and subsidiary faults. Adak Island itself sits near the Adak Canyon, a bathymetric feature associated with oblique subduction and transverse faulting that can localize seismic clusters.

Since 2000, four swarms have been identified in the immediate region, occurring in 2003 (one swarm), 2005 (one swarm), and 2006 (two swarms). These prior episodes demonstrate recurring swarm behavior in the area, typically characterized by rapid onset, high event rates over days, and a mix of low- to moderate-magnitude events without a single dominant mainshock. The 2007 swarm aligns with this pattern, featuring an initial energetic phase on 2 August that included the peak magnitude, followed by a gradual decline in both rate and maximum magnitude.

Event statistics from the first 100 detections reveal a classic swarm signature: numerous events clustered within the first 24 hours, interspersed with aftershock-like sequences following the larger shocks. Depths show modest variation, with many events between 10 km and 15 km, suggesting activity near the plate interface or within the overriding crust. No events exceeded magnitude 5.3, and the sequence produced no reported damage or tsunami, as expected for this offshore location and magnitude range.

The Aleutian arc remains one of the most seismically active regions on Earth. Ongoing monitoring by regional networks continues to document similar swarms, underscoring the importance of real-time analysis for distinguishing background subduction-related activity from potential precursors to larger events.

References

USGS Earthquake Catalog
Alaska Earthquake Center Annual Reports
NOAA Bathymetric Data for the Aleutian Region