Seismic Swarm S20210915.1: Analysis of Earthquake Activity Near Harding-Birch Lakes, Alaska

Seismic swarm S20210915.1 occurred in interior Alaska, centered 9 km northwest of Harding-Birch Lakes. The sequence began at 05:56 on 14 September 2021 and concluded at 09:20 on 18 September 2021, spanning 99 hours and 23 minutes. During this period, 63 earthquakes were recorded.

The swarm featured a range of event magnitudes and depths. The largest event reached magnitude 4.9 at a depth of 3 km on 14 September at 05:56:36. Subsequent activity included multiple events between magnitude 0.3 and 3.1, with depths varying from 0 km to 26 km. Notable later shocks included a magnitude 3.1 event at 8 km depth on 17 September at 23:33:19 and a magnitude 2.8 event at 6 km on 15 September at 02:33:21. Depths clustered predominantly between 0 and 10 km, indicating shallow crustal processes.

This swarm represents the second such episode in the region since 2000, following the previous swarm in 2011. The 2021 sequence exhibited a typical swarm pattern of clustered, moderate-magnitude events without a single dominant mainshock-aftershock progression.

Interior Alaska lies within a tectonically active zone influenced by the broader Pacific-North American plate interactions. The region experiences distributed seismicity associated with strike-slip faulting and crustal deformation, particularly along structures linked to the Denali Fault system. Historical records document recurrent earthquake activity tied to these faults, contributing to ongoing landscape evolution through uplift and basin formation.

The provided swarm parameters align with established patterns of intraplate seismicity in the area. Depths and magnitudes are consistent with shallow crustal release common in this part of Alaska.

References

SeismoSight internal classification data for swarm S20210915.1.
USGS Earthquake Hazards Program regional tectonic summaries.