Seismic Swarm S20250627.1: Activity Near Fort Bidwell, California
A seismic swarm designated S20250627.1 occurred 40 km east-northeast of Fort Bidwell in Modoc County, California. The sequence began at 07:48 on 26 June 2025 and concluded at 00:39 on 29 June 2025, spanning 64 hours and 50 minutes. During this interval, 38 earthquakes were recorded, with magnitudes ranging from 1.2 to 3.4 and focal depths predominantly between 0 and 9 km.
The swarm exhibited typical characteristics of clustered microseismicity, featuring an initial moderate event of magnitude 3.0 followed by a peak magnitude 3.4 event early on 27 June. Subsequent activity included multiple events above magnitude 2.0 distributed throughout the period, with many events clustered at depths of 4–6 km. Shallow events near 0 km depth appeared later in the sequence, while negative depth readings likely reflect location uncertainties common in sparse network coverage.
Fort Bidwell lies within the northwestern Basin and Range province, a region of active crustal extension. The local geology consists primarily of Miocene to Pliocene volcanic rocks of the Modoc Plateau, underlain by older metamorphic and granitic basement. The Surprise Valley Fault, a major normal fault striking northwest-southeast, bounds the area and accommodates ongoing extension at rates of approximately 1–2 mm per year. This tectonic setting produces both volcanic and tectonic seismicity, with historical earthquakes often linked to fluid migration or magmatic processes at depth.
Since 2000, twenty swarms have been documented in the immediate region. Notable prior episodes occurred in 2007 (one swarm), 2014 (ten swarms), 2015 (eight swarms), and 2022 (one swarm). These recurrent swarms suggest persistent structural weaknesses and possible hydrothermal influences within the volcanic terrain.
The 2025 swarm aligns with patterns observed in earlier sequences, where event rates peak within the first 24–48 hours before gradually declining. No damage or felt reports beyond the immediate vicinity were associated with the largest events, consistent with the modest magnitudes and rural setting.
References
United States Geological Survey Earthquake Catalog
California Geological Survey Regional Fault Maps
SeismoSight Internal Swarm Classification Database