Analysis of the August 2014 Earthquake Swarm East of Fort Bidwell, California

The earthquake swarm designated S20140819.1 occurred approximately 43 km east of Fort Bidwell in Modoc County, California. It began at 03:31 on 18 August 2014 and concluded at 00:26 on 21 August 2014, spanning 68 hours and 55 minutes. During this interval, 111 earthquakes were recorded. The events clustered in a region of the Modoc Plateau, part of the broader Basin and Range province where crustal extension drives normal faulting along north-northwest trending structures.

Analysis of the first 100 events reveals predominantly shallow focal depths, with the majority at 0–3 km and isolated occurrences reaching 6 km. Magnitudes ranged from 1.2 to 3.6, indicating a low-to-moderate energy release typical of swarm sequences rather than a mainshock-aftershock pattern. The largest event, magnitude 3.6, occurred on 19 August at 04:00:34. Temporal distribution showed peak activity in the early hours of 19 August, with multiple events exceeding magnitude 2.0 clustered between 00:00 and 06:00. Depths remained consistently shallow, suggesting rupture within brittle upper crustal layers influenced by local volcanic and sedimentary sequences.

The Modoc Plateau hosts extensive Quaternary basaltic and andesitic volcanism, with underlying fault systems accommodating regional extension rates of several millimeters per year. Historical records indicate sparse but recurrent seismicity in this northeastern California corner, where interaction between the Walker Lane shear zone and Cascade Range tectonics contributes to distributed deformation. The 2014 swarm represents the initial sequence identified in the SeismoSight classification since 2000, underscoring episodic swarm behavior in areas of low background seismicity.

Such swarms often arise from fluid migration or aseismic slip on minor faults, though precise triggering mechanisms require further geophysical investigation. No damage or felt reports beyond the immediate area were associated with this activity, consistent with the modest magnitudes and remote location. Ongoing monitoring by regional seismic networks continues to track strain accumulation along nearby structures, including the Surprise Valley Fault system.

References

USGS Earthquake Catalog
California Geological Survey Regional Fault Maps
SeismoSight Internal Swarm Database