Seismic Swarm S20151224.1: Analysis of Activity East of Fort Bidwell, California

Seismic swarm S20151224.1 occurred 45 km east of Fort Bidwell in Modoc County, California, within the northern Basin and Range extensional province. The sequence began at 01:16 UTC on 24 December 2015 and concluded at 03:58 UTC on 11 January 2016, spanning 434 hours and 41 minutes. During this interval, 467 earthquakes were recorded.

The first 100 events provide key insight into swarm dynamics. Activity initiated with a magnitude 1.9 event at 6 km depth. Magnitudes ranged from 0.7 to 4.4, with the largest shock occurring at 20:30 UTC on 24 December at 11 km depth. Depths clustered predominantly between 3 km and 12 km, consistent with shallow crustal faulting. Multiple events of magnitude 2.4–3.6 followed the mainshock, including a 3.6 at 10 km depth on 25 December at 02:36 UTC. Temporal distribution showed rapid succession in the first 24 hours, with events occurring at intervals of minutes, gradually decreasing in frequency. Depths remained stable across the early phase, indicating a consistent source volume without significant migration.

This swarm fits regional patterns of episodic earthquake clusters driven by fluid migration or aseismic slip along normal faults. The Surprise Valley Fault system, which bounds the Warner Mountains to the east, accommodates ongoing extension at rates of approximately 1–2 mm per year. Volcanic rocks of the Modoc Plateau, emplaced from the Miocene through the Holocene, overlie these structures and host geothermal features that may influence swarm triggering.

Historical records indicate 17 swarms in the region since 1 January 2000. Notable prior episodes include 10 swarms in 2014 and 7 in 2015, underscoring recurrent activity linked to the same tectonic setting.

References

• United States Geological Survey Earthquake Catalog
• California Geological Survey, Regional Geologic Maps
• University of Nevada, Reno, Nevada Seismological Laboratory Swarm Database