Seismic Swarm S20060824.1 Near Little Lake, California: Geological Context and Event Analysis

Seismic swarm S20060824.1 occurred approximately 15 km northeast of Little Lake in Kern County, California. The sequence began at 20:30 UTC on 23 August 2006 and concluded at 08:37 UTC on 1 September 2006, spanning 204 hours and 7 minutes. During this period, 89 earthquakes were recorded, with the majority exhibiting shallow focal depths between 0 and 6 km. The largest event reached magnitude 3.3 on 24 August 2006 at a depth of 0 km, followed by several events exceeding magnitude 2.0, including a magnitude 2.3 on 29 August. Event rates peaked during the first 48 hours, with subsequent activity showing a gradual decline. Magnitudes predominantly ranged from 0.2 to 1.6, consistent with typical swarm characteristics involving numerous small events rather than a dominant mainshock-aftershock pattern. The Little Lake region lies within the Eastern California Shear Zone, a zone of distributed right-lateral strike-slip faulting that accommodates a portion of the Pacific-North America plate boundary motion. This tectonic setting features active faults such as the Little Lake fault zone, which trends northwest-southeast and exhibits Quaternary slip. The area forms part of the broader Basin and Range province, where crustal extension interacts with shear deformation. Proximity to the Coso Volcanic Field, located roughly 20 km to the north, introduces additional complexity through geothermal activity and potential magmatic influences that can trigger fluid-driven seismicity. Seismic swarms in this region often result from interactions between tectonic stress, hydrothermal fluids, and minor volcanic processes. Depths recorded in the swarm align with the brittle-ductile transition zone influenced by elevated geothermal gradients. Historical data indicate recurrent swarm activity, with 22 swarms documented since 1 January 2000. Yearly occurrences include four in 2000, four in 2001, two in 2002, seven in 2004, one in 2005, and four in 2006, underscoring persistent seismic unrest. Analysis of event timing reveals clustering on 24–26 August and 28–30 August, suggesting episodic triggering possibly linked to pore-pressure diffusion. Depths remained consistently shallow, with 70 percent of events at or above 3 km, supporting a model of upper-crustal faulting modulated by fluids. No surface rupture was associated with the sequence, typical for low-magnitude swarms in this setting. This swarm contributes to understanding strain accumulation along the Eastern California Shear Zone, where geodetic measurements indicate slip rates of 6–12 mm per year. Continued monitoring aids in distinguishing tectonic from geothermal drivers of seismicity.

References

USGS Earthquake Catalog (historical seismicity data for California) California Geological Survey, Fault Activity Map of California Southern California Earthquake Data Center (regional swarm statistics)