Seismic Swarm Near Sierraville, California (2011-2012)

The region 2 km southwest of Sierraville in Sierra County, California, lies within the northern Sierra Nevada, near the transition to the Basin and Range province. This area features active normal faulting associated with regional extension, primarily along structures such as the Mohawk Valley Fault Zone. The Sierra Nevada experiences moderate seismicity driven by tectonic forces, with historical events linked to both volcanic and fault-related activity. Crustal thickness and geothermal influences contribute to occasional earthquake swarms, which differ from mainshock-aftershock sequences by lacking a dominant event.

SeismoSight recorded swarm S20111229.1 beginning at 00:21 on 29 December 2011 and concluding at 03:50 on 1 January 2012. Over 75 hours and 29 minutes, 89 earthquakes occurred. Magnitudes ranged from -0.6 to 2.1, with the majority below 1.0 and only a few reaching 1.5 or higher. Depths clustered predominantly between 29 and 32 km, though some shallower events occurred near 9-14 km and 27-28 km. Activity peaked on 29 and 30 December, featuring multiple events per hour during intense periods, followed by a sharp decline into early January.

The swarm displayed typical characteristics of low-magnitude, closely spaced events without a clear primary shock. Early activity on 29 December included several events near magnitude 1.3-1.6 at depths around 10-12 km, transitioning to deeper foci near 30 km as the sequence progressed. Later peaks on 30 December produced the two largest events at magnitude 2.1, both at 29 km depth. The pattern suggests distributed stress release along a fault segment rather than migration from a single point source.

Historically, seven swarms have been documented in the area since 2000, with one in 2001 and six in 2011. This concentration indicates episodic swarm behavior possibly tied to fluid migration or aseismic slip in the deep crust. Such sequences provide insights into local stress fields and may precede larger tectonic adjustments, though this particular swarm remained minor with no reported damage.

Geological records confirm the Sierra Nevada's long-term uplift and fault evolution since the Miocene, with Quaternary activity sustaining low-level seismicity. Updated monitoring by regional networks continues to track these patterns for improved hazard assessment.

References

USGS Earthquake Hazards Program regional seismicity reports.
California Geological Survey Sierra Nevada tectonic summaries.
SeismoSight internal swarm classification data.