Seismic Swarm S20140805.1 Near Fort Bidwell, California

Seismic swarm S20140805.1 occurred 43 km east of Fort Bidwell in Modoc County, California, within the northern Basin and Range province. This extensional tectonic setting features active normal faults, including segments of the Surprise Valley Fault system, which accommodate regional crustal stretching between the Sierra Nevada and the Colorado Plateau. The area lies near the transition to the Cascade volcanic arc, where Quaternary basaltic and andesitic volcanism has shaped the landscape, contributing to a structurally complex crust prone to distributed seismicity.

The swarm initiated at 10:25 UTC on 4 August 2014 and concluded at 18:19 UTC on 9 August 2014, spanning 127 hours and 54 minutes. During this interval, 82 earthquakes were recorded, with the majority exhibiting shallow focal depths between 0 and 4 km. Magnitudes ranged from 1.0 to 3.2, indicating low to moderate energy release consistent with fluid-driven swarm behavior rather than a single mainshock-aftershock sequence. Peak activity concentrated on 5 August, when events such as the magnitude 3.2 at 08:30 UTC and magnitude 2.9 at 22:44 UTC occurred amid numerous smaller tremors.

Temporal patterns reveal episodic bursts, with clusters of events separated by periods of relative quiescence. Most events were shallow, suggesting involvement of near-surface faults or hydrothermal processes common in this volcanic-influenced terrain. Depths occasionally reached 8–13 km, hinting at deeper crustal involvement along the regional fault network. No events exceeded magnitude 4.0, aligning with the historical pattern of modest swarm activity in the region.

Since 1 January 2000, only one swarm has been documented in this locale, underscoring the infrequent but recurrent nature of such sequences east of Fort Bidwell. The 2014 episode represents the sole recorded instance, highlighting limited background seismicity punctuated by short-lived swarms likely tied to local stress perturbations or fluid migration.

This swarm provides insight into the mechanics of intraplate extension, where small-magnitude events can delineate active fault segments without producing significant surface rupture. Ongoing monitoring by regional networks aids in distinguishing swarms from tectonic mainshocks, supporting hazard assessment in sparsely populated northeastern California.

References

USGS Earthquake Catalog
California Geological Survey, Regional Geologic Maps
USGS Professional Paper on Basin and Range Tectonics