Seismic Swarm S20201107.1 Near Big Lake, Alaska

SeismoSight recorded Swarm S20201107.1 beginning at 04:22 on 7 November 2020 and concluding at 12:37 on 13 November 2020. The sequence occurred 5 km north of Big Lake, Alaska, and comprised 150 earthquakes over 152 hours and 14 minutes.

The region lies within the Cook Inlet basin of south-central Alaska, where ongoing subduction of the Pacific plate beneath the North American plate generates frequent seismic activity. This tectonic setting produces intermediate-depth earthquakes along the Wadati-Benioff zone, with typical focal depths between 30 km and 60 km. The broader area has experienced notable events, including the magnitude 7.1 Anchorage earthquake of 30 November 2018, whose epicenter was approximately 20 km to the southeast.

Analysis of the first 100 events shows magnitudes ranging from 0.7 to 5.1, with the majority below 2.0. Depths clustered between 34 km and 52 km, consistent with the regional subduction environment. The sequence opened with a magnitude 5.1 event at 12:23 on 7 November at 41 km depth, followed within minutes by events of 3.7, 3.4, 3.0, 2.8, and 3.9. Subsequent activity included a magnitude 4.4 shock at 15:03 the same day at 43 km depth. Later events remained predominantly below magnitude 2.0, with depths stabilizing near 40 km.

Historical records since 1 January 2000 document only three prior swarms in the immediate vicinity: one each in 2005, 2009, and 2018. The low frequency of swarm-type sequences underscores the predominantly mainshock-aftershock character of regional seismicity.

The 2020 swarm illustrates typical subduction-related behavior, with an initial energetic phase giving way to lower-magnitude events over several days. Depths remained stable throughout, supporting an interpretation of activity along the plate interface rather than shallow crustal faults.

References
SeismoSight internal catalog, Swarm S20201107.1.
Alaska Earthquake Center regional tectonic summaries.