Seismic Swarm S20120110.1: Analysis Near Sierraville, California

Seismic swarm S20120110.1 was recorded 2 km southeast of Sierraville in Sierra County, California. The sequence began at 16:42 on 9 January 2012 and concluded at 09:29 on 16 January 2012, spanning 160 hours and 46 minutes. A total of 139 earthquakes were detected during this period. The first 100 events exhibited predominantly low magnitudes, ranging from -0.6 to 1.4, with the majority below 0.5. Depths clustered tightly around 32–34 km for most events, consistent with lower-crustal activity, although a small subset occurred at shallower levels near 10–11 km. Timing showed an initial dense cluster on 10 January followed by sustained, lower-rate activity through 12 January. No single dominant mainshock was present, characteristic of swarm behavior driven by distributed stress release rather than a classic foreshock–mainshock–aftershock pattern. Sierraville occupies the northern Sierra Nevada near the boundary with the Basin and Range province. This tectonic transition zone experiences distributed deformation linked to the Walker Lane belt, where right-lateral shear and normal faulting accommodate Pacific–North American plate motion. Regional seismicity often occurs along reactivated Mesozoic structures and Quaternary normal faults that bound the Sierra Valley graben. Lower-crustal depths observed in the swarm align with known rheological conditions in the Sierra Nevada, where elevated temperatures permit brittle failure at greater depths than in typical continental crust. Since 1 January 2000, ten swarms have occurred in the immediate area. Prior episodes were recorded in 2004 (one swarm), 2005 (one swarm), and 2011 (seven swarms), with the 2012 event representing the sole swarm that year. These recurrent swarms suggest persistent fluid migration or aseismic slip processes along deep fault zones. The 2012 swarm’s depth consistency and low-magnitude distribution provide insight into ongoing tectonic adjustment in the northern Sierra Nevada. Continued monitoring remains essential given the region’s proximity to populated valleys and infrastructure. Data sourced from SeismoSight internal classification. Regional geological context drawn from USGS Quaternary fault and seismic hazard assessments for the Sierra Nevada and Walker Lane.