Seismic Swarm S20150831.1: Analysis of Activity East of Fort Bidwell, California

Seismic swarm S20150831.1 occurred approximately 43 km east of Fort Bidwell in Modoc County, California, within the tectonically active Modoc Plateau. The sequence began at 12:22 on 30 August 2015 and concluded at 19:35 on 1 September 2015, spanning 55 hours and 13 minutes. During this period, 43 earthquakes were recorded, with magnitudes ranging from 0.7 to 3.3 and focal depths between 7 and 11 km.

The swarm exhibited typical characteristics of clustered seismic activity without a dominant mainshock. Early events on 30 August included magnitudes of 1.6 and 2.2, followed by peaks of 2.7 and 2.6. The largest shocks reached 3.3 on both 31 August at 09:59 and 22:52. Activity tapered on 1 September, ending with a 2.9 magnitude event. Depths remained consistent in the upper crust, suggesting rupture along shallow fault structures.

This swarm aligns with the region's established pattern of episodic seismicity. Since 2000, 14 swarms have been documented in the area, including 10 in 2014 and 4 in 2015. Such sequences often reflect distributed strain release rather than single-fault failure, common in zones of distributed extension.

Geologically, the location lies within the Basin and Range province, where Cenozoic extension has produced north-trending normal faults and volcanic edifices of the Modoc Plateau. The Surprise Valley fault system and associated structures accommodate regional deformation linked to Pacific–North American plate interaction. Quaternary basalts and andesites dominate surface geology, with underlying Mesozoic accreted terranes providing a heterogeneous basement that influences rupture patterns. Historical records indicate recurrent low-to-moderate seismicity, consistent with ongoing tectonic adjustment.

The 2015 swarm likely resulted from fluid migration or aseismic slip triggering successive failures along minor faults. No surface rupture or damage was associated, reflecting the modest energy release. Continued monitoring remains essential given the area's proximity to geothermal features and potential for future sequences.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey, Regional Geologic Maps
SeismoSight internal swarm classification data