Analysis of Earthquake Swarm S20210820.2 Near Little Lake, California

The earthquake swarm designated S20210820.2 occurred in a tectonically active region of eastern California. It began at 23:11 on 19 August 2021 and concluded at 13:17 on 8 September 2021, spanning 470 hours and 5 minutes. During this period, 212 earthquakes were recorded at a location 18 km east-southeast of Little Lake, California. This area lies within the Eastern California Shear Zone, a network of faults accommodating part of the motion between the Pacific and North American plates. The local geology features the Coso Volcanic Field, where geothermal activity and crustal extension contribute to elevated seismicity. Depths of events typically range from 1 to 13 km, consistent with shallow crustal faulting in this setting. Historical records indicate persistent swarm activity since 2000, with a total of 107 swarms documented through 2021. Annual counts show variability, including 4 swarms in 2000, 7 in 2004, 7 in 2010, 29 in 2019, and 24 in 2020, underscoring the region's recurrent seismic clustering. Examination of the first 100 events reveals predominantly low-magnitude activity, with the majority between 0.0 and 1.0 and only isolated events reaching 2.6. Depths cluster between 3 and 9 km for most occurrences, though some extend to 11–13 km. Temporal distribution shows initial high frequency on 20 August, followed by episodic bursts through late August, with events spaced from minutes to hours apart. No clear migration pattern emerges from the timing and depth data alone. Such swarms are characteristic of fluid-influenced faulting or stress transfer in geothermal and shear-zone environments. Continued monitoring remains essential given the proximity to populated areas and infrastructure.

References

SeismoSight internal swarm classification S20210820.2 USGS Earthquake Hazards Program regional reports on Eastern California Shear Zone and Coso Volcanic Field