Earthquake Swarm S20150811.1: Analysis of Seismicity Near Fort Bidwell, California

The earthquake swarm designated S20150811.1 occurred in a tectonically active region of northeastern California, approximately 44 km east of Fort Bidwell. This area lies within the Modoc Plateau, characterized by extensive volcanic and sedimentary rock formations resulting from Miocene to Quaternary volcanic activity. The regional geology features normal faulting associated with Basin and Range extension, including structures such as the Surprise Valley Fault, which accommodates ongoing crustal deformation. The swarm initiated at 15:47 on 10 August 2015 and concluded at 17:13 on 20 August 2015, spanning 241 hours and 25 minutes. During this period, 158 earthquakes were recorded. Analysis of the first 100 events reveals predominantly shallow focal depths between 4 km and 12 km, with the majority clustered around 8–10 km. Magnitudes ranged from 0.3 to 4.2, indicating low-to-moderate energy release typical of swarm sequences rather than a single mainshock-aftershock pattern. Early activity on 10 August included events of magnitude 2.3, 2.8, and 2.5 within the first two hours, followed by a sequence of smaller shocks. Activity intensified on 13 August, featuring the largest event of magnitude 4.2 at 11:31, accompanied by multiple magnitude 2+ events at depths of 8–11 km. Subsequent days showed continued but diminishing rates, with notable magnitudes including 3.5 on 14 August and 3.3 on 15 August. Depths remained consistent, suggesting a stable seismogenic zone within the upper crust. This swarm aligns with historical patterns in the region. Since 1 January 2000, thirteen swarms have been documented, with prior episodes occurring in 2014 (10 events) and three instances in 2015. Such recurrent swarms reflect the influence of fluid migration or magmatic processes in a volcanic province, where stress perturbations trigger clustered seismicity without producing surface rupture. The Modoc Plateau's seismic hazard stems from its position at the intersection of the Walker Lane belt and the Cascade Range volcanic arc. Updated geological mapping confirms Quaternary fault scarps and basaltic flows that record episodic extension rates of 1–3 mm per year. These conditions support the occurrence of diffuse, swarm-like seismicity rather than large, infrequent earthquakes. Overall, S20150811.1 exemplifies the background tectonic regime of northeastern California, where monitoring of depth distributions and magnitude-frequency relationships aids in distinguishing volcanic from purely tectonic drivers. Continued observation remains essential for refining hazard assessments in this sparsely populated but geologically dynamic area.

References

SeismoSight internal swarm classification records. USGS Earthquake Catalog for regional event verification.