Seismic Swarm S20100920.1: Analysis of Activity Near Anchorage, Alaska

A notable seismic swarm, designated S20100920.1, occurred approximately 15 km west-southwest of Anchorage, Alaska, from 21:24 UTC on 20 September 2010 to 14:53 UTC on 23 September 2010. Over 65 hours and 28 minutes, the event sequence registered 89 earthquakes, providing valuable data on regional tectonic behavior.

The swarm initiated with a magnitude 4.9 event at a depth of 45 km. Subsequent activity consisted primarily of smaller events, with magnitudes ranging from 0.7 to 2.7 and focal depths mostly between 37 km and 47 km. A secondary peak included a magnitude 2.7 quake at 38 km depth on 20 September at 22:45 UTC. Activity gradually declined, concluding with a magnitude 1.3 event at 40 km depth.

This sequence reflects typical swarm characteristics, featuring numerous low-magnitude events without a dominant mainshock-aftershock pattern. Depths clustered in the mid-crustal range align with known seismogenic zones in the area.

Anchorage lies within the tectonically active Cook Inlet region of south-central Alaska, part of the Aleutian subduction zone. Here, the Pacific Plate subducts northwestward beneath the North American Plate at rates of approximately 5–7 cm per year. This convergent margin generates frequent seismicity, including intraslab and interface earthquakes. The region's geology features accreted terranes, volcanic arcs, and sedimentary basins that amplify ground motion during events.

Historical records document significant earthquakes affecting Anchorage. The 1964 Great Alaska Earthquake, magnitude 9.2, originated about 120 km east of the city and remains one of the largest recorded, causing extensive damage through shaking and tsunamis. Smaller swarms and clusters have occurred periodically, often linked to stress transfer along the megathrust or crustal faults such as the Castle Mountain Fault.

The 2010 swarm fits within this framework of ongoing plate-boundary deformation. Its concentration at intermediate depths suggests possible involvement of the subducting slab, consistent with Wadati-Benioff zone activity. Updated monitoring by regional networks confirms continued low-level seismicity in the Cook Inlet area since 2010, underscoring persistent tectonic loading.

Such swarms contribute to hazard assessment by revealing stress patterns and potential precursors to larger events. Continued observation supports refined models of subduction dynamics in this high-risk zone.

References

USGS Earthquake Hazards Program reports on Alaska seismicity.
Alaska Earthquake Center historical catalogs.
SeismoSight internal swarm classification data.