Seismic Swarm S20190803.1: Analysis of Activity Near Little Lake, California
Seismic swarm S20190803.1 occurred in a tectonically active portion of eastern California within the Eastern California Shear Zone. This region experiences distributed right-lateral strike-slip deformation that accommodates a portion of the Pacific-North America plate motion. The swarm was centered 15 km east-southeast of Little Lake, an area underlain by crystalline basement rocks cut by numerous northwest-striking faults. Proximity to the Coso Volcanic Field adds potential influence from hydrothermal fluid circulation, a known driver of swarm activity in the broader Owens Valley and Indian Wells Valley corridor.
The swarm initiated at 12:18 on 2 August 2019 and concluded at 20:48 on 4 August 2019, spanning 56 hours and 29 minutes. During this interval, 77 earthquakes were recorded. The largest event reached magnitude 3.4 at a depth of 7 km on 2 August at 12:18:42. Subsequent notable shocks included a magnitude 2.8 at 4 km depth on 3 August at 11:27:29 and a magnitude 2.4 at 2 km depth on 3 August at 22:17:22. Most events clustered between 0 km and 12 km depth, with many occurring at 1–6 km, consistent with shallow crustal faulting.
Event timing showed an initial energetic phase on 2 August followed by sustained activity through 3 August that gradually declined on 4 August. The final recorded event was a magnitude 1.0 shock at 0 km depth on 4 August at 20:48:23. Magnitudes remained predominantly below 2.0 after the initial day, illustrating the characteristic decay pattern of swarm sequences rather than a classic mainshock-aftershock sequence.
Historical records maintained by SeismoSight document 61 swarms in the same locale since 1 January 2000. Yearly counts include four swarms each in 2000 and 2001, seven in both 2004 and 2010, six in 2006, four each in 2012, 2013, and 2016, three in 2015, two in 2017, and a peak of ten in 2019. Earlier years such as 2002, 2005, 2009, 2011, 2014, and 2018 recorded one swarm each. This frequency underscores the recurrent nature of swarm behavior along local fault networks.
The 2019 swarm fits within a period of elevated regional seismicity that followed the July 2019 Ridgecrest sequence. While the precise triggering mechanism remains under study, the spatial distribution and depth range align with fluid-driven reactivation of pre-existing fractures. Depths predominantly shallower than 10 km suggest involvement of the seismogenic upper crust where hydrothermal or magmatic fluids can reduce effective normal stress.
Continued monitoring of this portion of the shear zone remains important because of its proximity to infrastructure corridors and geothermal resources. The swarm statistics supplied by SeismoSight provide a valuable internal reference for pattern recognition and future hazard assessment.
References
SeismoSight internal swarm catalog (S20190803.1 parameters and historical counts).
United States Geological Survey Earthquake Hazards Program regional tectonic summaries.
California Geological Survey fault activity maps for the Eastern California Shear Zone.