Seismic Swarm S20140915.1: Analysis of Activity East of Fort Bidwell, California

The seismic swarm designated S20140915.1 occurred 42 km east of Fort Bidwell in northeastern California. It began at 17:57 on 14 September 2014 and concluded at 17:14 on 17 September 2014, spanning 71 hours and 16 minutes. During this interval, 44 earthquakes were recorded. This event represents the first swarm documented in the region since 2000.

The swarm exhibited a magnitude range from 0.5 to 3.5, with the largest event measuring 3.5 at a depth of 1 km on 15 September at 04:20:35. Depths remained predominantly shallow, clustered between 0 and 2 km, with isolated occurrences at 10 km. Multiple events exceeded magnitude 2.0, including 2.8, 2.9, 3.0, and additional 3.0 readings, distributed across the three-day period. Activity peaked on 15 September, with subsequent decline on 16 and 17 September. No single dominant mainshock preceded the sequence, consistent with swarm characteristics where events occur in clusters without clear foreshock-aftershock patterns.

Geologically, the location lies within the Modoc Plateau, a volcanic province influenced by Basin and Range extension. This tectonic setting features normal faulting and crustal thinning, producing shallow seismicity often linked to fluid migration or regional stress release. Historical records indicate limited swarm activity prior to 2014, underscoring the rarity of such clustered events in the area since the start of the millennium.

Insights from the sequence highlight its short duration and shallow focus, suggesting localized crustal processes rather than deeper magmatic involvement. The concentration of events within the first 24 hours, followed by tapering, aligns with typical swarm decay observed in extensional regimes. Depths near the surface imply potential interaction with near-surface structures, though no surface rupture was associated.

References

SeismoSight internal swarm classification S20140915.1
USGS earthquake catalog for regional context