Seismic Swarm Analysis: Little Lake, California, July 2011

A notable earthquake swarm, designated S20110728.1, occurred east of Little Lake in Inyo County, California. The sequence began at 02:22 on 28 July 2011 and concluded at 08:54 on 30 July 2011, lasting 54 hours and 32 minutes. During this period, 40 earthquakes were recorded, with magnitudes ranging from 0.1 to 2.6 and focal depths predominantly between 5 and 10 km.

The swarm exhibited typical characteristics of clustered seismicity in a tectonically active region. Early events on 28 July included several magnitude-2.0 shocks at depths of 6–8 km, followed by a peak magnitude 2.6 event at 23:06:12. Subsequent activity tapered through 29 and 30 July, with smaller events (magnitudes below 1.0) dominating later stages. Depths remained shallow overall, consistent with brittle failure in the upper crust.

The Little Lake area lies within the Coso Volcanic Field, part of the Eastern California Shear Zone. This region experiences distributed right-lateral shear between the Pacific and North American plates, accommodating approximately 20–25% of the total plate-boundary motion. The Coso field features Quaternary basaltic and rhyolitic volcanism, active geothermal systems, and extensive fault networks, including northwest-trending strike-slip faults. These structures facilitate episodic swarm activity driven by fluid migration and stress perturbations rather than mainshock-aftershock sequences.

Seismic swarms have been recurrent in this locale. Historical records since 2000 document 31 swarms, distributed as follows: four in 2000, four in 2001, one in 2002, seven in 2004, one in 2005, six in 2006, one in 2009, and seven in 2010. Such patterns reflect ongoing strain accumulation and release along the Walker Lane–Eastern California Shear Zone transition.

Insights from the 2011 swarm underscore its short duration and low maximum magnitude, typical of fluid-influenced clusters in volcanic or geothermal settings. Most events clustered within an 8 km depth band, suggesting a confined source volume. The absence of a dominant mainshock and the rapid onset of multiple similar-sized events align with swarm mechanics observed elsewhere in the Coso region.

References

• United States Geological Survey Earthquake Catalog
• California Geological Survey Regional Fault Maps
• Coso Volcanic Field Geothermal Studies, Naval Air Weapons Station China Lake Reports