Seismic Swarm S20151119.1: Analysis Near Fort Bidwell, California

Seismic swarm S20151119.1 was recorded 43 km east of Fort Bidwell, California. Activity began at 08:26 on 18 November 2015 and concluded at 06:06 on 25 November 2015. In 165 hours and 39 minutes, the swarm produced 137 earthquakes. This sequence represents typical swarm behavior, with events clustered tightly in both time and space without a dominant mainshock followed by decaying aftershocks.

Analysis of the first 100 events reveals magnitudes ranging from 0.7 to 4.2, with the majority below 2.0. Depths were concentrated between 5 km and 12 km, consistent with shallow crustal faulting. Notable larger events included a magnitude 4.2 at 01:40 on 19 November at 12 km depth, a magnitude 4.0 at 01:50 on the same day at 12 km, and several events of magnitude 3.0–3.4 later on 19 November. Smaller events occurred at shallower depths of 4–8 km toward the later portion of the initial 100-event window. The temporal distribution shows peak activity on 19 November, with 60+ events recorded that day alone, followed by a gradual decline.

This swarm aligns with the region's established pattern of episodic seismic clusters. Historical statistics since 1 January 2000 document 16 prior swarms in the immediate area. Earlier sequences occurred in 2014 (10 events) and 2015 (6 events), indicating recurrent activity along the same structural trends.

The location lies within the northern Basin and Range province, where east-west extension drives normal faulting along north-south trending structures. The Surprise Valley fault system and associated features accommodate regional strain and host both seismic swarms and geothermal manifestations. Depths of 5–12 km correspond to the brittle-ductile transition zone typical of this extensional regime. Volcanic history in the adjacent Modoc Plateau contributes to elevated heat flow that may influence swarm recurrence.

No damage or felt reports beyond the immediate region were associated with the larger events in this swarm. Continued monitoring remains important given the area's history of repeated swarms.

References

SeismoSight internal swarm classification S20151119.1
USGS Earthquake Catalog (regional tectonics)
California Geological Survey (Basin and Range province summaries)