Seismic Swarm S20111108.1 Near Sierraville, California

Seismic swarm S20111108.1 occurred 1 km west-southwest of Sierraville in Sierra County, California. The sequence began at 12:05 on 7 November 2011 and concluded at 19:17 on 9 November 2011, spanning 55 hours and 11 minutes. During this period, 43 earthquakes were recorded.

Event parameters show predominantly low-magnitude activity. Magnitudes ranged from -0.4 to 1.8, with the majority below 1.0. Depths clustered near 30 km, though several events occurred at shallower levels between 10 km and 18 km. The initial events on 7 November registered at depths of 30–32 km with magnitudes between -0.2 and 1.4. Activity continued steadily through 8 November, including multiple events above magnitude 1.0, such as a 1.5 at 13:45 and a 1.8 at 15:15. The final events on 9 November occurred at reduced depths of 10–11 km with negative magnitudes.

This swarm exhibits classic characteristics of microseismicity clusters: high event frequency over a short interval, limited maximum magnitude, and consistent focal depths. The temporal distribution indicates peak activity during the afternoon and evening of 8 November, followed by rapid decline.

Sierraville lies within the northern Sierra Nevada, near the transition to the Basin and Range province. The regional geology features Mesozoic granitic intrusions of the Sierra Nevada batholith overlain by Tertiary volcanic rocks. Active faulting occurs along northwest-trending normal and strike-slip structures associated with the Walker Lane belt. These faults accommodate dextral shear between the Pacific and North American plates. Seismicity in the area typically occurs at mid-crustal depths, consistent with the observed swarm depths.

Historical records indicate five swarms in the region since 1 January 2000. Prior episodes occurred in 2004 (one swarm) and 2011 (four swarms). The repeated occurrence of swarms suggests episodic strain release along local fault segments without progression to a mainshock–aftershock sequence.

References
SeismoSight internal swarm classification database
USGS Earthquake Catalog (events 2000–2011, Sierra County, CA)