Seismic Swarm S20150211.1 Near Fort Bidwell, California

Seismic swarm S20150211.1 was recorded 43 km east of Fort Bidwell in Modoc County, northeastern California. The sequence began at 00:37 on 11 February 2015 and concluded at 03:56 on 9 March 2015, spanning 627 hours and 19 minutes. During this period, 529 earthquakes were registered. Analysis of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from 0.3 to 2.9, with the majority falling between 0.8 and 1.7. Depths were shallow, concentrated between 4 km and 11 km, consistent with upper-crustal processes. Notable events included a magnitude 2.9 quake at 07:29 on 11 February and several magnitude 2.0–2.6 shocks later that day and into 12 February. The temporal distribution showed clusters of activity, particularly in the morning and evening hours of 11–12 February, followed by a gradual decline in frequency. The region forms part of the Modoc Plateau within the Basin and Range Province. This area experiences extensional tectonics driven by normal faulting along north-northwest-trending structures. Quaternary basaltic volcanism has shaped the landscape, with lava flows and cinder cones overlying older volcanic and sedimentary units. Seismic swarms here are commonly associated with fluid migration along faults or minor magmatic movement at depth. Historical records indicate ten swarms have occurred in the area since 1 January 2000. The first documented swarm took place in 2014, establishing a pattern of episodic, low-magnitude earthquake sequences in this tectonically active zone. Such swarms provide insight into the ongoing stress regime and potential volcanic-tectonic interactions characteristic of northeastern California’s geology. Continued monitoring supports improved understanding of regional hazard potential. References: SeismoSight internal classification data for swarm S20150211.1 USGS Earthquake Catalog and regional geologic maps of the Modoc Plateau