Seismic Swarm S20160813.1 Near Little Lake, California: Event Analysis and Regional Context

Seismic swarm S20160813.1 occurred in the region 23 km east of Little Lake, California, within the tectonically active Eastern California Shear Zone. The sequence began at 08:10 on 12 August 2016 and concluded at 18:13 on 14 August 2016, spanning 58 hours and registering 46 earthquakes. All events were shallow, with focal depths predominantly between 2 km and 4 km, and magnitudes ranging from -0.8 to 2.4. The largest event reached magnitude 2.4 at 02:47 on 13 August.

This swarm exemplifies typical characteristics of clustered seismicity in the area, featuring a gradual onset, multiple similar-magnitude events without a dominant mainshock, and rapid decay. Events clustered tightly in both time and space, with the majority occurring at 3 km depth. Such patterns often reflect fluid migration or minor crustal adjustments along pre-existing faults rather than large-scale tectonic release.

The Little Lake region lies at the southern margin of the Coso Volcanic Field in Inyo County. This area forms part of the broader Basin and Range extensional province, where northwest-trending dextral faults accommodate Pacific-North American plate motion. The Coso field itself hosts Quaternary rhyolitic domes and ongoing geothermal activity, with heat flow significantly above regional averages. Historical volcanic eruptions in the field date to the late Pleistocene, approximately 40,000 years ago, though no Holocene activity is recorded.

Seismicity in this sector has been monitored continuously since the late twentieth century. Since 2000, the area has experienced 45 documented swarms, with notable concentrations in 2004 (7 swarms), 2006 (6), 2010 (7), and 2012 (4). These recurrent swarms align with the presence of a shallow brittle-ductile transition influenced by elevated geothermal gradients. The 2016 event fits within this established pattern, occurring during a period of otherwise low background seismicity.

No damage or felt reports were associated with swarm S20160813.1, consistent with its modest magnitudes. Continued monitoring by regional networks remains essential for distinguishing swarm behavior from potential precursors to larger tectonic events along nearby fault systems such as the Little Lake fault zone.

References

USGS Earthquake Hazards Program regional seismicity reports (2016 data release).
California Geological Survey, Quaternary Fault and Fold Database.
Coso Volcanic Field geothermal assessments, U.S. Department of Energy (updated through 2023).