Seismic Swarm S20130812.1 Near Little Lake, California

Seismic swarm S20130812.1 occurred in a tectonically active portion of eastern California, centered 15 km northeast of Little Lake. The sequence began at 08:46 on 11 August 2013 and concluded at 22:19 on 13 August 2013, spanning 61 hours and 32 minutes. During this interval, 64 earthquakes were recorded, with the majority exhibiting magnitudes below 1.0 and focal depths between 0 and 6 km.

The swarm displayed typical characteristics of clustered microseismicity, featuring numerous events of low or negative reported magnitude alongside a single notable shock of magnitude 3.7 at 07:32 on 12 August. Shallow depths predominated, consistent with activity along near-surface fault strands. Event timing showed an initial sparse phase on 11 August, followed by a pronounced increase in frequency and a peak around midday on 12 August before tapering through 13 August.

Regional geology places the swarm within the Eastern California Shear Zone, where distributed right-lateral strike-slip faulting accommodates Pacific–North America plate motion. The Little Lake area lies near the intersection of the Little Lake Fault Zone and structures associated with the Coso Volcanic Field, a region known for recurrent earthquake swarms driven by fluid migration and geothermal processes. Historical records indicate 39 swarms in the vicinity since 1 January 2000, with notable activity in 2004 (7 swarms), 2006 (6), 2010 (7), and 2012 (4). The 2013 sequence represents the third swarm documented that year.

Such swarms contribute to long-term strain release without producing a single dominant mainshock. Their shallow, low-magnitude nature aligns with observations from prior episodes in the same locale, underscoring the persistent seismic hazard posed by the underlying fault network.

References

SeismoSight internal swarm classification database
USGS Earthquake Catalog (regional context)
California Geological Survey fault maps