Seismic Swarm S20050805.1: Analysis of Activity Southwest of Adak, Alaska

Seismic swarm S20050805.1 occurred in the central Aleutian Islands, approximately 116 km southwest of Adak, Alaska. The sequence began at 11:35 UTC on 4 August 2005 and concluded at 05:08 UTC on 6 August 2005, encompassing a total duration of 41 hours and 33 minutes. During this interval, 41 earthquakes were recorded, with magnitudes ranging from 1.7 to 3.8 and focal depths between 4 km and 67 km.

The largest event reached magnitude 3.8 at 19:12 UTC on 5 August 2005 at a depth of 23 km. Other notable shocks included a magnitude 3.3 event at the swarm onset (depth 12 km) and several magnitude 2.9 events distributed across the second day. Depths showed a broad distribution, with many events clustered between 12 km and 28 km, consistent with activity in the overriding plate and upper portions of the subducting slab.

This region 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. The resulting tectonic framework produces the Aleutian volcanic arc and accounts for the area’s persistently high seismicity. Adak itself sits on the Andreanof Islands segment, a portion of the arc that has hosted multiple great earthquakes, including the magnitude 8.6 event of 1957 and the magnitude 8.7 Rat Islands earthquake of 1965. These historic ruptures demonstrate the capacity for megathrust events along the plate interface.

Swarm activity in this setting often reflects localized stress adjustments along subsidiary faults or fluid migration within the subduction complex. Depths extending to 67 km suggest involvement of both crustal and uppermost mantle processes. The 2005 sequence did not produce surface rupture or significant aftershock migration beyond the initial cluster, remaining a self-contained episode of moderate-magnitude release.

Ongoing monitoring by the Alaska Earthquake Center and the USGS confirms that the central Aleutians continue to experience both isolated events and occasional swarms. Updated catalogs through 2023 show no comparable swarm precisely at this location since 2005, underscoring the episodic nature of such activity within an otherwise steadily deforming subduction environment.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center, University of Alaska Fairbanks
NOAA National Centers for Environmental Information tectonic summaries