Seismic Swarm S20140829.1: Analysis of Activity East of Fort Bidwell, California

Seismic swarm S20140829.1 was recorded 42 km east of Fort Bidwell in northeastern California. The sequence began at 13:44 on 28 August 2014 and concluded at 16:30 on 30 August 2014, spanning 50 hours and 46 minutes. During this interval, 36 earthquakes were registered, providing a clear example of clustered seismic behavior in the region.

The events displayed a range of magnitudes and focal depths. The sequence opened with a magnitude 2.4 event at 8 km depth. Subsequent activity included multiple events between magnitudes 1.0 and 2.8, with depths varying from surface level to 19 km. Notable larger shocks comprised a magnitude 3.8 at 2 km depth on 29 August at 08:46, a magnitude 3.1 at 1 km on the same day at 09:47, and a magnitude 3.0 at 1 km at 17:47. Later events included magnitudes 2.7 and 2.8 at depths of 9 km and 3 km, respectively. The final recorded event was a magnitude 1.3 at 17 km depth.

Northeastern California lies within the Basin and Range province, where crustal extension produces normal faulting along north-south trending structures. The Modoc Plateau, underlain by extensive basaltic and andesitic volcanic rocks, forms the local geologic framework. This volcanic terrain results from Miocene to Quaternary magmatism linked to regional tectonics and proximity to the Cascade volcanic arc. Seismicity in the area commonly reflects slip on range-bounding faults or minor adjustments within the volcanic edifice, occasionally accompanied by fluid migration that can trigger swarm-like sequences.

Historical records indicate that four swarms have occurred in the region since 1 January 2000, with S20140829.1 representing the earliest documented episode. Such swarms typically feature numerous small- to moderate-magnitude events without a single dominant mainshock-aftershock pattern, consistent with the observed distribution here.

The 2014 swarm illustrates characteristic features of swarm activity, including rapid onset, variable event depths, and a concentration of energy release within the first 36 hours. Magnitudes remained below 4.0, and no damage was reported. Depths clustered between 0 km and 9 km for most events, suggesting activity within the brittle upper crust.

References

SeismoSight internal swarm classification S20140829.1 dataset.
USGS Earthquake Catalog for regional tectonic context.
California Geological Survey geologic maps of the Modoc Plateau.