Seismic Swarm S20191018.2: Analysis of Activity Near Little Lake, California

The Little Lake region in Kern County, California, sits within the Mojave Desert portion of the Eastern California Shear Zone, a tectonically active area accommodating right-lateral shear between the Pacific and North American plates. Local fault systems, including strands associated with the Garlock Fault and nearby northwest-trending structures, contribute to frequent microseismicity. The zone experiences distributed deformation rather than slip on a single master fault, producing episodic earthquake swarms alongside occasional larger events. Swarm S20191018.2 began at 23:45 UTC on 17 October 2019 and concluded at 21:33 UTC on 8 November 2019. Over 525 hours and 47 minutes, 614 earthquakes were recorded 17 km east-southeast of Little Lake. The sequence displayed classic swarm characteristics: no single dominant mainshock, rapid onset of activity, and a gradual decline in event rate without a clear aftershock decay pattern. Analysis of the first 100 events reveals predominantly low-magnitude seismicity. Magnitudes ranged from 0.2 to 2.4, with the majority between 0.5 and 1.5. Depths clustered between 1 km and 10 km, indicating shallow crustal sources consistent with the regional fault architecture. Early activity on 18 October included several events near magnitude 1.8 at depths of 1–2 km, followed by a gradual migration toward slightly greater depths by 20–21 October. The largest event in this initial window reached magnitude 2.4 at 6 km depth on 21 October. Historical records document 73 swarms in the same locale since 1 January 2000. Annual counts show variability, with notable clusters in 2004 (7 swarms), 2006 (6), 2010 (7), and a marked increase to 22 swarms in 2019. This recent uptick aligns with elevated strain rates observed across the shear zone following the 2019 Ridgecrest sequence to the north. The 2019 activity underscores the region’s capacity for prolonged, diffuse seismicity driven by fluid migration or aseismic slip transients along interconnected fault networks. Such swarms contribute to long-term hazard assessment by illuminating active fault segments and stress transfer pathways. Continued monitoring remains essential given the proximity to infrastructure corridors and the historical precedent for swarm activity preceding or accompanying larger regional earthquakes. References SeismoSight internal swarm classification records USGS Earthquake Catalog for regional context