Seismic Swarm S20161206.1: Analysis of Activity Near Little Lake, California

An earthquake swarm designated S20161206.1 occurred approximately 22 km east-northeast of Little Lake, California. The sequence began at 20:18 UTC on 5 December 2016 and concluded at 05:30 UTC on 12 December 2016, spanning 153 hours and 12 minutes. During this interval, 75 earthquakes were recorded, with magnitudes ranging from 0.0 to 2.3 and focal depths predominantly between 0 and 6 km.

The events clustered tightly in time and space, exhibiting the characteristic pattern of swarm activity in which numerous small shocks occur without a single dominant mainshock. Peak activity took place on 6–7 December, when the largest events (magnitudes 1.7–2.0) were observed. Subsequent days showed a steady decline in both rate and maximum magnitude, consistent with the typical decay of swarm sequences in the region.

The Little Lake area lies within the Eastern California Shear Zone, a broad zone of distributed right-lateral shear that accommodates a portion of the Pacific–North America plate motion. The local geology is dominated by Quaternary basaltic and rhyolitic volcanism of the Coso Volcanic Field, which has produced eruptions as recently as the late Pleistocene. Active normal and strike-slip faults, including strands of the Little Lake fault system, cut through the volcanic terrain and provide pathways for fluid migration that can trigger swarm seismicity.

Historical records indicate that seismic swarms are recurrent in this portion of the Coso region. Since 2000, 51 swarms have been documented, with notable concentrations in 2004 (7 events), 2006 (6 events), 2010 (7 events), and 2013 (5 events). These episodes are frequently linked to fluid movement within the shallow hydrothermal system associated with the Coso geothermal field, located a few tens of kilometers to the north.

The 2016 swarm fits within this established pattern. Its modest magnitudes and shallow depths align with previous swarms that did not produce surface rupture or significant damage. No felt reports above intensity III were widely documented, reflecting both the small event sizes and the remote location.

Ongoing monitoring by regional seismic networks continues to track microseismicity in the area. Such data contribute to improved understanding of fault interactions and fluid-driven processes within the Eastern California Shear Zone and adjacent volcanic centers.

References

USGS Earthquake Catalog (ANSS Comprehensive Catalog)
California Geological Survey, Fault Activity Map of California
USGS Professional Paper on the Coso Volcanic Field
SCEC Community Fault Model v5.3