Seismic Swarm S20201010.1: Analysis of Activity Near Little Lake, California

The region 20 km east-southeast of Little Lake, California, sits within the Eastern California Shear Zone, a network of active strike-slip faults that accommodates a significant portion of Pacific-North American plate motion. This tectonically complex area features northwest-trending faults and experiences frequent small-magnitude seismicity, often in swarm-like clusters. Depths of events typically range from shallow crustal levels of 1–11 km, reflecting brittle failure in the upper crust influenced by both regional shear and local volcanic or geothermal processes associated with the nearby Coso Volcanic Field.

Seismic swarms have been a recurring feature in this locale. Historical records since 2000 document 101 such swarms, with notable increases in frequency during 2019 (29 swarms) and 2020 (21 swarms). These patterns align with elevated background seismicity following the 2019 Ridgecrest earthquake sequence, which included magnitude 6.4 and 7.1 mainshocks and triggered widespread aftershock activity across the shear zone.

Swarm S20201010.1 began at 05:06 on 9 October 2020 and concluded at 12:03 on 12 October 2020, spanning 78 hours and 57 minutes. During this interval, 59 earthquakes were recorded. Magnitudes remained low, with the largest event reaching 2.2; most fell between 0.0 and 1.9. Focal depths clustered between 3 and 11 km, indicating activity confined to the seismogenic crust without significant deeper involvement.

Event distribution showed temporal clustering, with higher rates on 9–10 October, including multiple events near magnitude 1.0–2.0. Later days featured sparser activity tapering to negative magnitudes by 12 October. Depths varied modestly, with several events at 4–9 km suggesting migration or activation along a localized fault segment.

This swarm exemplifies typical swarm behavior in the Eastern California Shear Zone, characterized by diffuse, low-magnitude sequences lacking a dominant mainshock-aftershock pattern. Such events contribute to ongoing strain release and may illuminate fault interactions in a region where shear zone deformation continues to evolve post-2019.

References
USGS Earthquake Hazards Program catalog data for California seismicity.
California Geological Survey reports on the Eastern California Shear Zone.
SeismoSight internal swarm classification records for event parameters.