Seismic Swarm S20080502.1: Analysis of Activity Near Adak, Alaska

A significant seismic swarm, designated S20080502.1, occurred west of Adak, Alaska, beginning at 01:33 UTC on 2 May 2008 and concluding at 16:55 UTC on 21 May 2008. Over 471 hours and 21 minutes, the sequence registered 1079 earthquakes. The swarm initiated with a magnitude 6.6 event at 14 km depth, followed by a dense cluster of aftershocks. This activity unfolded in a tectonically active segment of the Aleutian arc, providing a clear example of swarm behavior in a subduction setting.

The region lies along 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 oblique subduction produces the Aleutian Islands and drives both megathrust earthquakes and volcanic activity. Adak Island sits near the Andreanof Islands segment, a portion of the arc known for recurrent seismicity. Shallow crustal depths, typically less than 20 km in the provided sequence, align with the brittle upper plate and the megathrust interface.

Historical records document multiple large events in the vicinity, including the 1957 magnitude 8.6 Andreanof Islands earthquake and the 1986 magnitude 7.9 event. These episodes demonstrate the zone’s capacity for both great earthquakes and prolonged aftershock sequences. Modern monitoring by the Alaska Earthquake Center and USGS confirms ongoing background seismicity, with swarms occasionally occurring due to stress transfer or fluid migration along the plate interface.

Examination of the first 100 events reveals a classic mainshock-aftershock decay pattern. The initial magnitude 6.6 shock was followed within minutes by events in the magnitude 3.0–3.7 range at depths of 4–9 km. Subsequent activity showed a rapid decline in maximum magnitudes, with most events falling between 1.5 and 2.9. Depths remained predominantly shallow, clustering between 4 and 13 km, consistent with aftershock relocation within the overriding plate and near the subduction interface. A small number of events reached depths of 16–17 km, suggesting limited activity on deeper portions of the fault system. The temporal distribution indicates the highest rate of occurrence in the first several hours, tapering steadily thereafter.

Overall, the swarm exemplifies typical post-mainshock relaxation in a convergent margin environment. The combination of a moderate-to-large trigger event and a high number of smaller aftershocks reflects efficient stress redistribution along pre-existing structures in the Aleutian subduction zone.

References

USGS Earthquake Catalog Alaska Earthquake Center Annual Reports Global CMT Catalog NOAA National Centers for Environmental Information