Seismic Swarm S20040807.1: Analysis of Activity Near Little Lake, California

Seismic swarm S20040807.1 occurred 12 km northeast of Little Lake, California, from 10:35 on 6 August 2004 to 05:11 on 15 August 2004. Over 210 hours and 35 minutes, the swarm produced 101 earthquakes. The sequence exemplifies the persistent microseismicity that characterizes this portion of the Eastern California Shear Zone.

The Little Lake region lies within the transition between the Sierra Nevada and the Basin and Range province. Active northwest-striking faults accommodate dextral shear between the Pacific and North American plates. Nearby Coso Volcanic Field adds a component of geothermal fluid migration that can modulate swarm behavior through pore-pressure changes. Depths recorded during the swarm clustered between 0 and 6 km, consistent with the shallow brittle crust typical of the area.

Examination of the first 100 events shows a rapid onset followed by a gradual decline in rate. Initial magnitudes reached 1.8 within the first hour, with the largest event of the analyzed sequence measuring 2.9 on 12 August. Most events remained below magnitude 1.0, and focal depths stayed shallow, rarely exceeding 5 km. A secondary peak on 12 August included several events above magnitude 1.5, illustrating the swarm’s episodic character rather than a classic mainshock-aftershock decay.

Regional historical data indicate that earthquake swarms are common. Between 1 January 2000 and the end of 2004, thirteen swarms were documented in the same locale. Yearly counts were four in 2000, four in 2001, two in 2002, and three in 2004. This recurrence underscores the area’s elevated background seismicity driven by both tectonic loading and local hydrothermal processes.

The 2004 swarm did not produce damage or felt reports beyond the immediate vicinity, consistent with the small magnitudes involved. Continued monitoring by regional networks remains essential for distinguishing purely tectonic swarms from those potentially influenced by geothermal exploitation at Coso.

References

SeismoSight internal swarm catalog, Swarm S20040807.1 parameters.
USGS Earthquake Catalog, regional seismicity 2000–2004.
California Geological Survey, Fault Activity Map of California.