Seismic Swarm S20130124.1: Analysis of Activity Near Coleville, California

Seismic swarm S20130124.1 occurred approximately 9 km west of Coleville in Mono County, California, within the tectonically active Walker Lane belt. This region lies at the western margin of the Basin and Range province, where right-lateral shear and normal faulting accommodate Pacific-North American plate motion. The area features Quaternary faults and is influenced by the nearby Sierra Nevada frontal fault system, contributing to recurrent low-to-moderate seismicity.

The swarm initiated at 08:21 UTC on 24 January 2013 and concluded at 05:42 UTC on 26 January 2013, spanning 45 hours and 20 minutes. During this interval, 61 earthquakes were recorded. Event depths ranged primarily from 0 to 12 km, with the majority clustered between 4 and 9 km, consistent with shallow crustal faulting in the region. Magnitudes varied from 0.4 to a peak of 4.0, the latter occurring at 23:25 UTC on 24 January at 4 km depth. Other notable events included a 3.6 magnitude shock at the swarm's onset and several events exceeding magnitude 2.0 distributed throughout the first day.

Swarms of this type typically lack a dominant mainshock-aftershock sequence and instead reflect distributed strain release along fault networks or fluid migration in the crust. In the Coleville area, such episodes align with historical patterns of episodic activity. Since 2000, six prior swarms have been documented in the immediate vicinity: single swarms in 2000, 2003, 2005, and 2007, plus two in 2010. These events underscore the region's persistent seismic productivity without large-magnitude releases.

The temporal distribution showed highest rates during the initial 24 hours, with 38 events on 24 January tapering to 22 on 25 January and a single event on 26 January. Depths remained stable across the sequence, suggesting a consistent source volume. No surface rupture or significant ground deformation was associated, typical for swarms below magnitude 5.

This activity provides insight into the mechanics of swarm behavior in transtensional regimes. Repeated small-magnitude events indicate stress transfer along en echelon faults rather than a single locked segment. Monitoring such swarms aids in refining probabilistic hazard assessments for the eastern Sierra Nevada transition zone.

References

SeismoSight internal classification for swarm S20130124.1 parameters and statistics.
USGS Earthquake Hazards Program regional tectonic summaries for Mono County, California.
California Geological Survey Quaternary fault database for Walker Lane belt characteristics.