Earthquake Swarm S20010810.1: Seismic Activity Near Plumas Eureka, California

The earthquake swarm designated S20010810.1 occurred in the northern Sierra Nevada region of California, centered 2 km west-northwest of Plumas Eureka. This sequence began at 16:20 on 10 August 2001 and concluded at 12:13 on 12 August 2001, spanning 43 hours and 53 minutes during which 59 earthquakes were recorded.

The Sierra Nevada range forms part of the tectonically active western margin of North America, where the interaction between the Pacific and North American plates generates ongoing crustal deformation. The Plumas Eureka area lies within a zone of distributed faulting associated with the broader Walker Lane belt, characterized by right-lateral shear and normal faulting. Geological formations in the vicinity include Mesozoic granitic intrusions of the Sierra Nevada batholith overlain by Cenozoic volcanic and sedimentary units. Historical seismicity in this portion of the range includes moderate events linked to both tectonic strain accumulation and possible magmatic influences from distant volcanic centers.

The swarm initiated with a magnitude 0.8 event at a depth of 6 km. Activity intensified rapidly, featuring a magnitude 5.2 earthquake at 20:19 on 10 August at 5 km depth. Subsequent events included multiple shocks between magnitudes 1.8 and 3.0 clustered at depths of 9 to 18 km. Notable later activity encompassed a magnitude 4.0 event on 11 August at 12:01, located at 5 km depth. Depths throughout the sequence ranged primarily from 3 km to 18 km, consistent with shallow crustal processes typical of the region.

Event rates peaked in the hours immediately following the magnitude 5.2 mainshock, with numerous aftershocks of magnitude 2.0 or less occurring at intervals of minutes. The sequence displayed classic swarm characteristics, lacking a single dominant mainshock-aftershock decay pattern and instead maintaining elevated activity over two days. Magnitudes remained predominantly below 3.0 after the initial peak, tapering to smaller events by the swarm's termination.

Such swarms contribute to understanding regional strain release in the northern Sierra Nevada, where background seismicity reflects both plate-boundary forces and local structural complexities. Updated monitoring by regional networks continues to refine models of fault behavior in this area, supporting hazard assessments for nearby communities.

References SeismoSight internal classification for Swarm S20010810.1 USGS Earthquake Hazards Program regional tectonic summaries California Geological Survey Sierra Nevada fault database