Seismic Swarm Analysis: Little Lake, California, December 2010

The seismic swarm designated S20101202.1 occurred 13 km northeast of Little Lake, California, within the tectonically active Eastern California Shear Zone. This region lies at the transition between the Basin and Range Province and the Mojave Desert, where distributed right-lateral strike-slip faulting accommodates Pacific-North America plate motion. The local geology features Quaternary alluvium overlying Mesozoic granitic basement rocks intruded by younger volcanic units associated with the nearby Coso Volcanic Field. Geothermal activity in the Coso area contributes to elevated heat flow, which can influence fluid-driven seismicity along pre-existing faults.

The swarm initiated at 07:55 UTC on 1 December 2010 and concluded at 20:11 UTC on 2 December 2010, spanning 36 hours and 16 minutes. During this interval, 30 earthquakes were recorded. Event magnitudes ranged from -0.2 to 3.2, with the majority occurring at shallow depths between 1 km and 4 km. Notable activity clustered around midday and evening on 1 December, including a 2.6 magnitude event at 12:16 UTC and two events exceeding magnitude 3.0 at approximately 20:53 UTC. Subsequent smaller events continued into the following day, tapering by late evening.

This swarm fits within a documented pattern of seismic activity in the area. Since 1 January 2000, 30 swarms have been identified near Little Lake. Annual counts include four swarms each in 2000 and 2001, one in 2002, seven in 2004, one in 2005, six in 2006, one in 2009, and six in 2010. Such episodic clustering reflects the influence of fluid migration and stress interactions along the Little Lake Fault zone and adjacent structures.

Seismic swarms in this setting typically lack a single dominant mainshock-aftershock sequence and instead feature numerous events of similar size. Depths predominantly under 5 km align with the brittle-ductile transition influenced by local geothermal gradients. The 2010 swarm's temporal evolution shows an initial burst followed by sustained low-magnitude activity, consistent with pore-pressure diffusion models observed in other geothermal-adjacent regions.

Ongoing monitoring by regional networks continues to track microseismicity, providing data for refined hazard assessments in this portion of Kern County. The combination of tectonic loading and hydrothermal processes maintains elevated background rates compared to surrounding stable blocks.

References

USGS Earthquake Catalog
California Geological Survey Regional Fault Maps
Coso Geothermal Field Seismicity Studies (peer-reviewed literature)