Seismic Swarm S20111002.1: Analysis of Activity Near Sierraville, California

A seismic swarm designated S20111002.1 was recorded north of Sierraville in Sierra County, California. The sequence began at 21:43 UTC on 1 October 2011 and concluded at 14:08 UTC on 16 October 2011, spanning 352 hours and 24 minutes. During this period, 382 earthquakes were detected. The swarm occurred in a region of the northern Sierra Nevada characterized by active tectonics associated with the broader Pacific-North American plate boundary. This area lies within the transition zone between the Sierra Nevada block and the Basin and Range province, where extensional forces contribute to seismicity along distributed faults. Historical records indicate limited swarm activity in the vicinity since 2000, with only three documented episodes: one event in 2004, one in 2005, and the 2011 sequence. Analysis of the first 100 events reveals predominantly microseismic activity. Magnitudes ranged from -1.0 to 1.5, with the majority below 0.5. Depths clustered tightly around 33–35 km for most events, though several shallower occurrences were noted between 3 km and 6 km. The initial event registered magnitude 0.2 at 33 km depth. Subsequent events maintained consistent deep focal depths until isolated shallower signals appeared on 2 October and again on 7–8 October. This pattern suggests a primary source volume at mid-crustal levels with limited upward migration. The temporal distribution showed higher event rates in the first 48 hours, followed by a gradual decline. No events exceeded magnitude 2.0 within the examined subset, consistent with swarm behavior rather than a classic mainshock-aftershock sequence. Such swarms in the Sierra Nevada are often linked to fluid migration or minor strain release along pre-existing structures. Geological context for the Sierraville area includes Mesozoic granitic intrusions of the Sierra Nevada batholith overlain by Cenozoic volcanic and sedimentary units. Regional faulting reflects ongoing deformation, with potential influence from the nearby Mohawk Valley fault system. Updated seismic monitoring by regional networks continues to track background seismicity in this zone. References
USGS Earthquake Catalog (events 2000–2023)
California Geological Survey, Regional Geologic Maps
SeismoSight internal swarm classification S20111002.1