Seismic Swarm S20010517.1: Analysis of Activity Near Little Lake, California

Seismic swarm S20010517.1 was recorded southwest of Little Lake, California, beginning at 19:49 on 17 May 2001 and concluding at 19:56 on 22 May 2001. The sequence lasted 120 hours and 7 minutes, during which 230 earthquakes were detected. The events occurred at a location approximately 19 km southwest of Little Lake in a region known for tectonic activity within the Eastern California Shear Zone.

Analysis of the first 100 events reveals a rapid onset with an initial magnitude 3.0 earthquake at 6 km depth. Subsequent activity included multiple events clustered between magnitudes 1.0 and 2.5, predominantly at depths of 5 to 8 km. Notable larger events comprised a magnitude 3.5 at 20:47 on 17 May at 4 km depth, a magnitude 4.2 at 21:53 at 7 km depth, and a magnitude 4.1 at 22:56 also at 7 km depth. Depths across the sequence generally ranged from 2 to 12 km, with the majority centered around 7 km. Magnitudes declined after the initial hours, shifting toward smaller events below magnitude 2.0 while maintaining a steady rate of occurrence through the evening of 17 May and into 18 May.

The swarm represents one of only two such sequences documented in the region since 1 January 2000, with the prior swarm occurring in 2000. This pattern underscores episodic seismic behavior rather than continuous high-rate activity.

The Little Lake area lies in eastern California near the transition between the Sierra Nevada and the Basin and Range Province. The local geology features active strike-slip faulting associated with the Eastern California Shear Zone, which accommodates a portion of the Pacific-North American plate motion. Nearby, the Coso Volcanic Field contributes to the tectonic setting through geothermal systems and Quaternary volcanism. Historical seismicity in the broader Owens Valley and Coso region reflects both tectonic extension and localized volcanic influences, with fault systems such as the Little Lake fault zone playing a role in strain release.

This 2001 swarm aligns with the area's established seismic character, where swarms often occur without producing significant surface rupture. Depths of the recorded events are consistent with brittle failure in the upper crust typical of the region.

References

USGS Earthquake Hazards Program catalog
California Geological Survey regional fault maps
SeismoSight internal swarm classification records