Seismic Swarm S20040224.2 Near Little Lake, California: Geological Context and Event Analysis

The seismic swarm S20040224.2 occurred 16 km northeast of Little Lake, California, beginning at 00:16 on 24 February 2004 and ending at 20:59 on 25 February 2004. In 44 hours and 43 minutes, 36 earthquakes were recorded. Magnitudes ranged from 0.3 to 1.9, with focal depths mostly between 0 and 6 km and occasional negative values consistent with very shallow or surface-proximal events. Activity clustered in several short bursts, including multiple events near 06:38 on 24 February and around 10:00 on 25 February, with the largest shock (magnitude 1.9) occurring at 20:27 on 24 February.

This pattern exemplifies typical swarm behavior in the region, where sequences lack a dominant mainshock and instead feature distributed, low-magnitude events over hours to days. Such swarms are common in areas experiencing fluid migration along faults or minor magmatic influence, allowing stress to be released gradually without producing a single large rupture.

The Little Lake area lies within the tectonically active Eastern California Shear Zone, part of the broader Basin and Range Province. Right-lateral strike-slip faulting dominates, accommodating a portion of the Pacific-North America plate motion. Proximity to the Coso Volcanic Field adds geothermal and volcanic components that can modulate seismicity through hydrothermal fluid movement and minor crustal heating. The local geology features Quaternary basaltic and rhyolitic volcanics overlying older metamorphic and granitic basement, intersected by numerous normal and strike-slip faults.

Seismic swarms have been recurrent in this setting. Since 1 January 2000, ten swarms have been documented in the immediate region, distributed as four in 2000, four in 2001, and two in 2002. These episodes underscore ongoing strain accumulation and release along the shear zone without escalation to larger mainshock-aftershock sequences.

No surface rupture or significant damage was associated with S20040224.2, consistent with the modest magnitudes involved. Continued monitoring of swarm frequency and migration patterns remains essential for understanding long-term seismic hazards in this portion of eastern California.

References
SeismoSight internal swarm classification records
USGS Earthquake Catalog and regional fault maps
California Geological Survey Quaternary fault database