Seismic Swarm S20200521.1: Analysis of Activity Near Little Lake, California

Seismic swarm S20200521.1 was recorded in the region 21 km east of Little Lake, California. The sequence began at 01:37 on 20 May 2020 and concluded at 04:59 on 24 May 2020, spanning 99 hours and 21 minutes. During this period, 97 earthquakes were detected, providing a detailed record of clustered seismic activity in a tectonically dynamic zone.

The Little Lake area lies within the Eastern California Shear Zone, a network of faults accommodating significant right-lateral shear between the Pacific and North American plates. This setting features shallow crustal deformation influenced by both strike-slip faulting and nearby volcanic processes associated with the Coso Volcanic Field. Historical seismic patterns in the region reflect episodic swarm activity driven by fluid migration and stress transfer along fault segments, rather than mainshock-aftershock sequences.

The swarm's temporal distribution showed the highest event density in the initial 48 hours, with magnitudes ranging from -0.5 to 2.2. Depths were predominantly shallow, concentrated between 1 km and 13 km, consistent with brittle failure in the upper crust. Notable events included a magnitude 2.2 earthquake on 21 May at 14:16:32 UTC at 2 km depth, followed closely by several smaller events within minutes. Subsequent days exhibited declining frequency, with isolated events up to magnitude 1.6 recorded through 24 May.

Geological context indicates that such swarms are characteristic of the broader Owens Valley and Mojave Desert transition zone. Since 2000, 94 swarms have occurred in the area, with notable yearly counts including 29 in 2019 and 14 in 2020. Earlier years showed lower but consistent activity: 7 swarms each in 2004 and 2010, 6 in 2006, and 4 each in 2000, 2001, 2012, 2013, and 2016. This long-term record underscores recurring strain accumulation and release along regional fault systems.

Insights from the event parameters reveal a predominance of microseismicity, with over 90 percent of events below magnitude 1.0. Depth variations suggest multiple fault strands or fluid pathways activated sequentially. The absence of a dominant mainshock aligns with typical swarm behavior in this volcanic-adjacent terrain, where pore-pressure changes can trigger diffuse seismicity without large ruptures.

Overall, swarm S20200521.1 exemplifies the ongoing seismic hazard in the Little Lake vicinity, where monitoring supports improved understanding of fault mechanics and volcanic-tectonic interactions.

References

SeismoSight internal swarm classification database.
USGS Earthquake Catalog for regional fault and historical context.
California Geological Survey reports on Eastern California Shear Zone tectonics.