Seismic Swarm S20191007.1: Analysis of Activity Near Little Lake, California

Seismic swarm S20191007.1 was recorded in the region 15 km east of Little Lake, California. The sequence began at 20:37 on 6 October 2019 and concluded at 12:38 on 8 October 2019, spanning approximately 40 hours and encompassing 48 earthquakes.

Event magnitudes remained predominantly low, with the largest reaching 2.5. Depths ranged between 1 km and 12 km, consistent with shallow crustal activity typical of the area. Notable events included a magnitude 2.3 earthquake on 8 October at 12:32 and several magnitude 1.6 shocks distributed across 7 October. Smaller events (magnitudes 0.0–0.9) constituted the majority, reflecting the swarm’s diffuse character rather than a single mainshock-aftershock pattern.

The Little Lake area lies within the Eastern California Shear Zone, a broad region of right-lateral strike-slip faulting that accommodates part of the relative motion between the Pacific and North American plates. This tectonic setting produces frequent small-to-moderate earthquakes and episodic swarms. Historical records since 2000 document 71 swarms in the vicinity, with notable yearly counts including 20 in 2019, 7 in 2010, and 7 in 2004. These swarms often occur without producing significant surface rupture or damage, yet they provide valuable data on local stress conditions and fault interactions.

The October 2019 swarm fits established patterns observed in the shear zone, where clusters of events can be triggered by fluid migration, aseismic slip, or stress perturbations from distant larger earthquakes. Depths predominantly between 2 km and 10 km align with the brittle-ductile transition zone in this part of the Mojave Desert crust.

Continued monitoring of such sequences contributes to improved understanding of regional seismic hazard. While individual swarms rarely exceed moderate magnitudes, their cumulative effect on fault loading warrants ongoing observation by networks such as those operated by the U.S. Geological Survey and the California Geological Survey.

References

SeismoSight internal swarm catalog (S20191007.1 parameters and historical statistics).
U.S. Geological Survey Earthquake Hazards Program regional tectonic summaries.
California Geological Survey fault and seismicity reports for the Eastern California Shear Zone.