Seismic Swarm S20190905.2: Analysis of Activity Near Little Lake, California

Seismic swarm S20190905.2 occurred in a tectonically active zone 18 km east-southeast of Little Lake, California. The sequence began at 19:11 on 4 September 2019 and concluded at 05:44 on 8 September 2019, spanning 82 hours and 32 minutes. During this period, 79 earthquakes were recorded, with magnitudes ranging from 0.2 to 2.7 and focal depths predominantly between 1 km and 13 km.

The swarm exhibited typical characteristics of clustered microseismicity, featuring numerous small events without a dominant mainshock. Notable events included a magnitude 2.4 earthquake at 8 km depth on 5 September at 17:33:52 and a magnitude 2.7 event at 3 km depth on 6 September at 06:49:35. Activity remained shallow overall, consistent with brittle failure in the upper crust of this region.

The Little Lake area lies within the Eastern California Shear Zone, where distributed right-lateral strike-slip faulting accommodates a portion of the Pacific-North America plate motion. This zone features numerous northwest-trending faults that interact with the Garlock Fault to the south and the Sierra Nevada frontal fault system to the north. Local geology includes Quaternary alluvium overlying Mesozoic basement rocks, with nearby volcanic features associated with the Coso Volcanic Field contributing to elevated heat flow and fracture permeability.

Historical records indicate persistent swarm activity in the broader area. Since 1 January 2000, 66 swarms have been documented. Yearly counts show variability, with peaks in 2019 (15 swarms), 2010 (7), 2004 (7), and 2006 (6). Earlier years such as 2002, 2005, and 2009 recorded only single events each. This pattern reflects episodic strain release along minor fault segments rather than steady background seismicity.

The September 2019 swarm fits within this established framework, occurring months after the July 2019 Ridgecrest sequence that ruptured nearby segments of the Little Lake Fault Zone. Depths and magnitudes align with prior swarms, underscoring the region's capacity for short-lived clusters of low-magnitude events.

Such swarms provide valuable data on fault connectivity and stress transfer in the shear zone. Continued monitoring supports improved understanding of seismic hazards in this transitional tectonic setting between the Basin and Range and the Mojave Desert.

References

• SeismoSight internal swarm classification records (S20190905.2 parameters and historical swarm statistics, 2000–2019).
• United States Geological Survey Earthquake Catalog for regional context on the Eastern California Shear Zone.