Seismic Swarm S20201003.1 Near Ridgecrest, California

The seismic swarm designated S20201003.1 was recorded 12 km northeast of Ridgecrest in Kern County, California. It began at 02:27 UTC on 2 October 2020 and concluded at 23:01 UTC on 27 October 2020, spanning 620 hours and 33 minutes. During this interval, 320 earthquakes were detected. Analysis of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from -0.1 to 3.4, with the largest shock occurring at 12:32 UTC on 2 October. Depths were shallow, concentrated between 0 km and 11 km, consistent with the brittle upper crust in this portion of the Mojave Desert. Early events clustered in the initial 48 hours, including multiple magnitude-1+ shocks on 2–4 October, followed by a gradual decline in both frequency and peak magnitude through 9 October. No events exceeded magnitude 3.4 within the examined subset, indicating a swarm dominated by microseismicity rather than a mainshock-aftershock sequence. The Ridgecrest area lies within the Eastern California Shear Zone, a network of northwest-trending right-lateral strike-slip faults that accommodate a portion of Pacific–North American plate motion. The 2019 Ridgecrest earthquake sequence (Mw 6.4 foreshock and Mw 7.1 mainshock) ruptured previously unmapped segments of the Little Lake Fault system, producing widespread stress redistribution that continues to influence afterslip and triggered seismicity. The October 2020 swarm occurred within the same aftershock volume, suggesting ongoing post-seismic relaxation along subsidiary faults. Historical records since 1 January 2000 document 101 swarms in the immediate region. Activity increased markedly after 2019, with 29 swarms recorded that year and 22 in 2020. Earlier decades showed lower rates, typically fewer than seven swarms annually. This acceleration aligns with the elevated background seismicity that followed the 2019 mainshock. The swarm’s shallow focal depths and low magnitudes are typical of fluid-assisted or aseismic-slip-driven sequences observed in the post-2019 stress field. Continued monitoring remains essential given the proximity to the Garlock Fault and the potential for stress transfer to other structures within the shear zone. References
USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey, Quaternary Fault and Fold Database
SeismoSight internal swarm classification S20201003.1