Seismic Swarm S20020318.1: Analysis of Activity Near Adak, Alaska

Seismic swarm S20020318.1 occurred 16 km east-northeast of Adak, Alaska, beginning at 04:43 on 17 March 2002 and concluding at 13:35 on 19 March 2002. Over 56 hours and 52 minutes, the sequence produced 75 earthquakes, with magnitudes ranging from 0.5 to 4.6 and focal depths primarily between 6 km and 32 km. The largest event reached magnitude 4.6 at 21 km depth on 18 March at 00:02, followed by additional events of magnitude 3.6 and 3.2 that clustered within the first 24 hours.

The temporal distribution showed peak activity on 18 March, with events occurring at intervals of minutes during several bursts. Depths remained shallow to intermediate, consistent with crustal processes above the subducting slab. Magnitudes stayed predominantly below 2.5 after the initial mainshock, indicating a typical swarm decay pattern without a single dominant aftershock sequence.

Adak lies within the central Aleutian Islands, part of the Aleutian volcanic arc formed by oblique subduction of the Pacific Plate beneath the North American Plate at rates of approximately 6–7 cm per year. This convergent margin generates frequent earthquake swarms through a combination of plate interface slip, crustal faulting, and magmatic fluid migration. The region experiences high seismicity, with the USGS recording thousands of events annually across the arc.

Historically, the Adak area has hosted multiple swarms linked to both tectonic and volcanic sources. Notable episodes occurred in 1956 and 1996, often preceding or accompanying volcanic unrest at nearby volcanoes such as Great Sitkin and Kanaga. These sequences typically feature low-to-moderate magnitudes and shallow-to-intermediate depths, reflecting stress accumulation and release along the megathrust and overlying crustal faults.

The S20020318.1 swarm aligns with this pattern, likely representing episodic slip on secondary faults or fluid-driven fracturing within the overriding plate. No surface rupture or significant volcanic activity was associated with the sequence.

Key Events Summary

The sequence initiated with a magnitude 2.0 event at 16 km depth. Subsequent activity included a magnitude 3.2 shock at 11 km depth on 17 March at 23:29 and the magnitude 4.6 main event early on 18 March. Later notable shocks comprised magnitudes 3.6 and 3.1 on 18 March at depths of 13 km and 11 km, respectively, followed by a magnitude 2.9 event near the swarm’s end on 19 March.

Geological Implications

Such swarms contribute to long-term strain release along the Aleutian subduction zone, reducing the potential for larger locked-zone ruptures. Continued monitoring by regional networks helps refine models of plate coupling and seismic hazard in this high-risk corridor.

References

USGS Earthquake Catalog (Adak region events 2002)
Alaska Earthquake Center annual reports
Tectonic framework of the Aleutian arc, Journal of Geophysical Research (updated syntheses through 2023)