Seismic Swarm S20121024.1 Near Little Lake, California

The seismic swarm designated S20121024.1 occurred in a tectonically active portion of eastern California, centered 23 km east-northeast of Little Lake. It began at 15:14 on 23 October 2012 and concluded at 20:55 on 29 October 2012, spanning 149 hours and 40 minutes. During this interval, 80 earthquakes were recorded, all of low magnitude and predominantly shallow focal depth.

The Little Lake region lies within the Eastern California Shear Zone, a broad zone of distributed right-lateral shear that accommodates a portion of the relative motion between the Pacific and North American plates. This area features numerous northwest-trending strike-slip faults and is influenced by ongoing Basin and Range extension. Nearby volcanic centers, including the Coso Volcanic Field, contribute to elevated heat flow and occasional fluid-driven seismicity. Historical records indicate that earthquake swarms are a recurrent feature here, often linked to fluid migration along fault networks rather than mainshock-aftershock sequences driven by a single large rupture.

Since 1 January 2000, 35 swarms have been documented in the vicinity. Yearly counts include four events each in 2000 and 2001, two in 2002, seven in 2004, one in 2005, six in 2006, one in 2009, seven in 2010, one in 2011, and two in 2012. These episodes underscore the persistent low-level seismic productivity of the region.

Within swarm S20121024.1, event magnitudes ranged from 0.0 to 1.8. The largest shocks (magnitudes 1.7–1.8) occurred on 24 October at shallow depths of 0–1 km. Most events clustered between 0 and 6 km depth, with isolated deeper occurrences reaching 16 km. Negative depth values in the catalog likely reflect minor location uncertainties near the surface. Activity peaked on 24 October, when more than 20 events were recorded, including a sequence of magnitude 1.0–1.8 shocks between 13:32 and 16:26. Subsequent days showed declining rates, with sporadic events continuing until the swarm’s termination.

Such swarms typically exhibit rapid onset followed by gradual decay without a dominant mainshock. The spatial concentration and shallow depths observed are consistent with fluid-pressure perturbations along pre-existing fractures in the shear zone. No damage or felt reports of significance were associated with this episode, reflecting the small energy release.

Continued monitoring of the Little Lake area remains important for understanding strain accumulation along regional faults and potential interactions with nearby geothermal resources. Updated seismic catalogs from regional networks provide the basis for ongoing analysis of swarm recurrence and characteristics.

References
SeismoSight internal swarm classification database.
United States Geological Survey Earthquake Catalog (regional California data).
California Geological Survey, Fault Activity Map of California.