Seismic Swarm S20110914.2 Near Sierraville, California
A notable earthquake swarm, designated S20110914.2, occurred 2 km west-northwest of Sierraville in Sierra County, California. The sequence began at 10:42 on 14 September 2011 and concluded at 20:51 on 15 September 2011, spanning 34 hours and 9 minutes. During this period, 42 earthquakes were recorded, with magnitudes ranging from -0.7 to 1.4 and focal depths predominantly between 29 and 35 km.
The swarm initiated with a magnitude 1.0 event at 32 km depth. Subsequent activity consisted primarily of microearthquakes below magnitude 1.0, interspersed with occasional events reaching 0.4 to 0.6. Two larger shocks of magnitude 1.4 occurred at shallower depths of 29 km, one at 14:18 on 14 September and another at 03:55 on 15 September. Depths remained consistent around 32–33 km for most events, indicating a concentrated source volume within the mid-crust.
This activity aligns with the broader tectonic framework of the northern Sierra Nevada. The region occupies the transition zone between the stable Sierra Nevada block and the extensional Basin and Range province to the east. Faulting here is influenced by the Sierra Nevada frontal fault system and secondary structures associated with the Walker Lane shear zone. Although overall seismicity rates are moderate, the area experiences occasional swarms linked to fluid migration or minor stress adjustments along pre-existing fractures.
Historical records since 2000 indicate swarm events are uncommon in this locale. Only two prior swarms have been documented: one in 2004 and another in 2005. The 2011 sequence therefore represents a rare recurrence, providing additional data on the low-level seismic behavior of the northern Sierra Nevada.
The tight clustering of events in both time and space, combined with the absence of a clear mainshock-aftershock pattern, confirms the swarm classification. Depths near 30 km suggest involvement of the lower crust, where ductile-brittle transitions can localize small-magnitude failure. No damage or felt reports were associated with these events given their small sizes.
References
USGS Earthquake Catalog
California Geological Survey regional fault maps
SeismoSight internal swarm database