Seismic Swarm S20190918.1 Near Little Lake, California
Seismic swarm S20190918.1 was recorded 24 km east-southeast of Little Lake, California. Activity began at 18:48 UTC on 17 September 2019 and concluded at 07:41 UTC on 2 October 2019, spanning 348 hours and 53 minutes. A total of 471 earthquakes were detected during this interval.
The region lies within the Eastern California Shear Zone, a tectonically active belt characterized by right-lateral strike-slip faulting and distributed deformation between the Pacific and North American plates. This zone accommodates a portion of the relative plate motion and hosts numerous Quaternary faults. The Coso Volcanic Field, located nearby, contributes geothermal activity and elevated heat flow that influence local seismicity patterns. Earthquake depths in the area commonly range from 1 to 10 km, reflecting brittle failure within the upper crust.
Analysis of the first 100 events reveals predominantly shallow focal depths between 1 and 10 km. Magnitudes ranged from 0.1 to 2.9, with the largest event (M2.9) occurring on 20 September 2019 at 20:33 UTC at 9 km depth. Most events clustered between M0.5 and M1.5, consistent with typical swarm behavior where energy release occurs through numerous small shocks rather than a dominant mainshock. Temporal distribution showed an initial increase in rate after the M2.0 and M2.1 pair on 20 September, followed by sustained activity through 21 September.
Historical records indicate frequent swarm activity in the region. Since 1 January 2000, 68 swarms have been documented, with notable yearly counts including 18 in 2019, 7 each in 2004 and 2010, and 5 in 2006. This pattern underscores the persistent seismic productivity of the Little Lake area, driven by ongoing tectonic strain and possible fluid migration associated with nearby volcanic and geothermal systems.
The swarm's characteristics align with regional norms, where low-magnitude, clustered events predominate without producing significant surface rupture. Continued monitoring remains essential given the proximity to infrastructure and the potential for larger triggered events in the shear zone.
References
USGS Earthquake Catalog
California Geological Survey fault database
SeismoSight internal swarm classification S20190918.1