Seismic Swarm S20191005.1 Near Little Lake, California

The seismic swarm designated S20191005.1 began at 01:59 on 5 October 2019 and concluded at 18:05 on 8 October 2019. Centered 16 km east-southeast of Little Lake in California, the sequence produced 147 earthquakes over 88 hours and six minutes. Analysis of the first 100 events reveals predominantly low-magnitude activity, with values ranging from 0.0 to 2.6 and the majority below 1.0. Depths remained shallow, concentrated between 1 km and 10 km, consistent with brittle failure in the upper crust. The initial 24 hours accounted for the highest event rate, including the largest shock of magnitude 2.6 at 15:22 on 5 October. Subsequent days showed a gradual decline in frequency while maintaining similar magnitude and depth distributions. This swarm occurred within the Eastern California Shear Zone, a region of distributed right-lateral strike-slip faulting that accommodates a portion of the Pacific-North America plate motion. The local geology features Quaternary alluvium overlying Mesozoic basement rocks cut by northwest-trending faults. Little Lake lies near the intersection of the Garlock Fault and the Airport Lake Fault zone, where strain accumulates episodically. Historical records document 57 swarms in the area since 1 January 2000, with notable increases in frequency during 2010 (six events) and especially 2019 (23 events). These swarms typically exhibit short durations and low maximum magnitudes, often linked to fluid migration or stress triggering along secondary faults. The sequence followed closely after the July 2019 Ridgecrest earthquake sequence, whose magnitude-7.1 mainshock occurred only 6 km from the swarm center. Post-mainshock viscoelastic relaxation and afterslip likely contributed to the elevated swarm activity observed in October. Depths recorded in the first 100 events align with the seismogenic zone documented for the region, rarely exceeding 12 km. No events reached magnitudes capable of widespread damage, though the cumulative energy release reflects ongoing tectonic adjustment. References:
USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey Fault Activity Map
Southern California Earthquake Data Center (scedc.caltech.edu)