Seismic Swarm Analysis: Fort Bidwell Region, California (July 2015)

A seismic swarm designated S20150716.1 occurred east of Fort Bidwell in northeastern California. The sequence began at 14:20 on 15 July 2015 and concluded at 23:23 on 19 July 2015, spanning 105 hours and registering 258 earthquakes. The epicentral area lies within the Modoc Plateau, a volcanic province characterized by extensional tectonics associated with Basin and Range deformation.

The first 100 events exhibited magnitudes predominantly between 0.6 and 4.6, with the majority clustered below 2.5. Depths ranged from 0 to 12 km, averaging near 7 km, consistent with shallow crustal faulting typical of the region. Notable activity included a 4.6 magnitude event at 10 km depth on 16 July at 01:52, followed by additional shocks of 4.0 and multiple 3.4 magnitude events within the subsequent hour. Subsequent events showed a gradual decline in peak magnitudes while maintaining similar depth distributions.

This swarm represents one of eleven documented sequences in the area since 1 January 2000. Earlier activity included ten swarms in 2014 and one in 2015 prior to the July event. Such episodic clustering aligns with the tectonic setting near the California-Nevada-Oregon border, where normal faults accommodate regional extension.

The Modoc Plateau hosts Quaternary basaltic and andesitic volcanism linked to the ancestral Cascade arc and subsequent back-arc spreading. Fault systems, including segments of the Surprise Valley fault zone, provide pathways for fluid migration that can trigger swarm activity through pore-pressure changes. Historical records indicate recurrent low-to-moderate seismicity, with no surface rupture associated with the 2015 sequence.

Monitoring by regional networks confirms the swarm's parameters, offering insight into the area's ongoing tectonic adjustment without evidence of magmatic involvement. Continued observation supports improved understanding of swarm recurrence in this sparsely populated but geologically dynamic portion of the western United States.