Seismic Swarm S20101008.1: Analysis of Activity Near Adak, Alaska

Seismic swarm S20101008.1 occurred 111 km east-southeast of Adak, Alaska, beginning at 03:32 UTC on 8 October 2010 and concluding at 12:58 UTC on 11 October 2010. Over 81 hours and 25 minutes, the sequence produced 184 earthquakes. The first 100 events, recorded primarily on 8 October, displayed magnitudes ranging from 1.8 to 6.0, with focal depths between 0 km and 36 km. The largest event reached magnitude 6.0 at 03:49:10 UTC on 8 October at 27 km depth, followed by a magnitude 5.7 shock at 04:19:14 UTC at 6 km depth. Additional notable events included magnitudes of 4.7, 4.6, and multiple 4.5 shocks clustered in the initial hours. The swarm exhibited a rapid onset with several events exceeding magnitude 4.0 within the first hour, followed by a gradual decline in both frequency and magnitude. Depths remained predominantly in the 5–30 km range, consistent with crustal and upper-mantle processes. No events in the provided sequence surpassed magnitude 6.0, and activity tapered after the first 24 hours. The Adak region lies within the central Aleutian Islands, part of the tectonically active Pacific Ring of Fire. Here, the Pacific Plate subducts beneath the North American Plate along the Aleutian Trench at rates of approximately 6–7 cm per year. This subduction drives intense seismicity, including both shallow crustal events and deeper Wadati-Benioff zone earthquakes. The local geology features volcanic arcs, fault systems, and accreted terranes shaped by millions of years of plate convergence. Historical records indicate that seismic swarms are infrequent in this sector. Since 1 January 2000, only one prior swarm has been documented, occurring in 2008. The 2010 sequence therefore represents the second such episode in the instrumental era for the immediate area. The Aleutian subduction zone has produced great earthquakes, such as the 1957 magnitude 8.6 event and the 1965 magnitude 8.7 Rat Islands earthquake, underscoring the region’s capacity for both swarm-type and megathrust activity. This swarm provides insight into transient stress changes along the plate interface and overlying crust. The concentration of shallow events suggests possible fluid migration or aseismic slip triggering brittle failure. Continued monitoring by regional networks remains essential for distinguishing swarm behavior from foreshock sequences preceding larger mainshocks. References
SeismoSight internal swarm catalog (S20101008.1)
Alaska Earthquake Center historical seismicity records
USGS Earthquake Hazards Program – Aleutian subduction zone summary