Seismic Swarm S20150914.1 Near Fort Bidwell, California

Seismic swarm S20150914.1 was recorded beginning at 16:41 on 13 September 2015 and concluding at 22:44 on 15 October 2015. The events were centered 43 km east of Fort Bidwell in Modoc County, California. Over 774 hours and 2 minutes, a total of 545 earthquakes were registered.

The Fort Bidwell region lies within the Modoc Plateau, part of the broader Basin and Range extensional province near its transition to the Cascade volcanic arc. This area features Miocene to Pleistocene basaltic and andesitic volcanic rocks overlying older sedimentary and metamorphic basement. Active normal faulting and regional extension contribute to background seismicity, with occasional swarms linked to fluid migration or minor magmatic processes at depth.

Since 1 January 2000, 15 swarms have occurred in the vicinity. Earlier episodes include 10 swarms in 2014 and 5 in 2015 prior to S20150914.1. These sequences typically involve low-to-moderate magnitude events clustered over days to weeks.

Analysis of the first 100 events reveals a rapid onset with the largest shock reaching magnitude 4.7 at 13:55 on 14 September 2015. Magnitudes ranged from 0.8 to 4.7, with the majority between 1.0 and 2.5. Focal depths clustered between 5 km and 12 km, averaging approximately 8 km. The initial 24 hours accounted for the highest rate of activity, including multiple events above magnitude 2.0. Subsequent events showed a gradual decline in both frequency and peak magnitude while maintaining similar depth ranges.

This pattern is consistent with swarm behavior observed in volcanic and extensional terranes of northeastern California, where sequences often lack a single dominant mainshock. No surface rupture or significant damage was associated with the swarm.

References

• SeismoSight internal swarm classification records
• USGS earthquake catalog for northeastern California
• California Geological Survey regional geologic maps (Modoc Plateau)