Seismic Swarm Analysis: November 2011 Event Near Sierraville, California

A notable earthquake swarm, internally classified as S20111111.1, occurred 2 km southwest of Sierraville in Sierra County, California. The sequence began at 14:58 on 10 November 2011 and concluded at 18:27 on 15 November 2011, spanning 123 hours and 28 minutes. During this period, 128 earthquakes were recorded.

Analysis of the first 100 events reveals predominantly microseismic activity. Magnitudes ranged from -0.7 to 2.2, with the majority falling below 1.0. Only a small subset exceeded magnitude 1.5, including events of 2.2, 2.0, and 1.7. Focal depths clustered tightly around 27–34 km, suggesting activity within the mid-to-lower crust typical of the Sierra Nevada block.

Temporal distribution showed peak occurrence within the first 48 hours, followed by a gradual decline. Early events included several negative-magnitude detections at depths near 30 km, transitioning to slightly shallower activity (around 27–28 km) during stronger phases. This pattern aligns with fluid-driven swarm behavior often observed in tectonically active continental margins.

The Sierraville region occupies the northern Sierra Nevada, where the crust experiences distributed deformation linked to the broader Pacific–North American plate boundary. Local fault systems, including strands of the Sierra Nevada frontal fault zone and nearby Walker Lane structures, accommodate right-lateral shear and extension. Seismicity in this area frequently manifests as swarms rather than mainshock–aftershock sequences, reflecting possible contributions from magmatic or hydrothermal fluids at depth.

Historical records since 1 January 2000 document seven swarms in the immediate vicinity. These occurred in 2004 (one swarm), 2005 (one swarm), and five separate swarms in 2011, indicating episodic clustering. Such recurrence underscores the region’s persistent low-to-moderate seismic productivity without large-magnitude mainshocks.

The 2011 swarm contributed to ongoing characterization of crustal properties beneath the northern Sierra Nevada. Depths exceeding 27 km imply relatively cool, brittle conditions extending into the lower crust, consistent with geophysical models of the Sierra Nevada batholith. No surface rupture or significant damage was associated with the sequence.

References

• SeismoSight internal swarm catalog (S20111111.1 parameters)
• USGS Earthquake Catalog (regional seismicity 2000–present)
• California Geological Survey, Sierra Nevada fault compilation
• Waldhauser & Schaff (2008) updated double-difference catalogs for northern California