Seismic Swarm S20191002.1 Near Ridgecrest, California: Analysis of Early Events
Seismic swarm S20191002.1 occurred in the Eastern California Shear Zone, approximately 11 km east-northeast of Ridgecrest in Kern County. This region forms part of the Mojave Desert and lies at the intersection of multiple active fault systems, including strands of the Garlock Fault and northwest-trending faults associated with the Walker Lane belt. The area experiences distributed right-lateral shear between the Pacific and North American plates, producing frequent small-to-moderate earthquakes.
The swarm initiated at 08:09 UTC on 1 October 2019 and concluded at 22:07 UTC on 5 October 2019, spanning 109 hours and 58 minutes. During this interval, 127 earthquakes were recorded. Analysis of the first 100 events reveals predominantly low-magnitude activity, with magnitudes ranging from -0.1 to 2.2. Depths clustered between 0 and 11 km, consistent with shallow crustal faulting typical of the region.
Early events on 1 October remained below magnitude 2.0, with the largest reaching 1.8 at a depth of 2 km. Activity intensified slightly on 2 October, including a magnitude 2.2 event at 5 km depth and several magnitude 1.6–1.7 shocks. Subsequent days showed continued low-level seismicity, punctuated by isolated magnitude 1.7–2.1 events. Depths showed no systematic migration, remaining distributed across the upper crust.
This swarm fits within the broader seismic history of the Ridgecrest area. Since 2000, ten swarms have been documented in the vicinity, with eight occurring in 2019 alone. The elevated rate followed the July 2019 Ridgecrest sequence, which included a magnitude 6.4 foreshock and a magnitude 7.1 mainshock that ruptured multiple fault segments. Post-mainshock aftershocks and triggered swarms reflect ongoing stress redistribution along the complex fault network.
Such swarms provide insight into fault interaction and fluid involvement in the shallow crust. Continued monitoring remains essential for understanding how these sequences relate to the long-term strain accumulation across the Garlock and nearby faults.
References
SeismoSight internal swarm classification database
USGS Earthquake Catalog (queried October 2019)
California Geological Survey fault maps