Seismic Analysis of Swarm S20200613.1 in Eastern California
Earthquake swarm S20200613.1 was recorded in the region 19 km east-southeast of Little Lake, California. The sequence began at 09:35 on 12 June 2020 and concluded at 18:05 on 27 June 2020, spanning 368 hours and 30 minutes. During this interval, 334 earthquakes were detected.
The swarm occurred within the Eastern California Shear Zone, a tectonically active belt characterized by distributed right-lateral strike-slip faulting. This zone accommodates a portion of the Pacific-North American plate boundary motion north of the Garlock Fault. The Little Lake area lies near the transition between the Sierra Nevada frontal fault system and the Coso Volcanic Field, where geothermal activity and Quaternary volcanism contribute to elevated seismicity. Historical records indicate persistent microseismicity driven by both tectonic loading and fluid migration in fractured crystalline basement rocks.
The first 100 events of the swarm exhibited magnitudes predominantly between 0.0 and 3.1, with the majority clustered at depths of 2–9 km. Early activity on 12 June included events at 09:35:10 (M 1.0, 8 km), 19:12:18 (M 1.5, 4 km), and 21:07:45 (M 1.9, 2 km). A notable M 3.1 event occurred at 06:45:06 on 13 June at 2 km depth, followed by several M 2.4 shocks later that day. Depths remained shallow throughout the initial phase, consistent with activity along upper-crustal faults. Event frequency peaked within the first 48 hours before gradually declining.
Regional seismic history shows repeated swarm episodes. Since 1 January 2000, 97 swarms have been documented in the broader area. Annual counts include 4 in 2000, 4 in 2001, 1 in 2002, 7 in 2004, 1 in 2005, 6 in 2006, 1 in 2009, 7 in 2010, 1 in 2011, 4 in 2012, 4 in 2013, 1 in 2014, 3 in 2015, 4 in 2016, 2 in 2017, 1 in 2018, 30 in 2019, and 16 in 2020. These episodes reflect episodic strain release along distributed faults, often without a single dominant mainshock.
The 2020 swarm fits established patterns of short-duration, low-magnitude sequences typical of the Coso-Little Lake corridor. Such activity provides data for refining fault models and assessing background seismicity rates in this portion of the shear zone.
References
- SeismoSight internal swarm catalog (S20200613.1 parameters and event list)
- USGS Earthquake Catalog (regional historical swarm statistics 2000–2020)