Seismic Swarm S20200403.1 Near Little Lake, California

Seismic swarm S20200403.1 occurred in a tectonically active zone 21 km east-southeast of Little Lake, California. The sequence began at 09:52 on 2 April 2020 and concluded at 16:29 on 5 April 2020, spanning 78 hours and 37 minutes. During this interval, 67 earthquakes were recorded, with magnitudes ranging from -0.3 to 2.3 and focal depths between 2 km and 11 km. Most events clustered at shallow depths of 3–8 km, consistent with brittle failure in the upper crust. The swarm exhibited typical characteristics of episodic seismic activity in this region, including a rapid onset followed by a gradual decline in event rate. The largest event reached magnitude 2.3 on 4 April at 22:54, accompanied by several events above magnitude 1.5. Smaller events dominated the catalog, reflecting the swarm’s low-to-moderate energy release. The Little Lake area lies within the Eastern California Shear Zone, a network of right-lateral strike-slip faults that accommodate part of the Pacific–North American plate boundary motion. This zone includes the Little Lake Fault and intersects with the Coso Volcanic Field to the north, where geothermal activity and Quaternary volcanism contribute to elevated crustal temperatures and fluid circulation. These conditions promote swarm-type seismicity rather than mainshock–aftershock sequences. Historical records document 91 swarms in the broader region since 2000. Activity peaked in 2019 with 29 swarms, followed by 12 in 2020. Earlier years showed lower frequencies, with notable clusters in 2004 (7), 2010 (7), and 2012–2016 (multiple events per year). Such recurrent swarms indicate persistent tectonic strain and fluid involvement along fault networks. The April 2020 swarm aligns with this pattern, underscoring the area’s ongoing seismic productivity. Depths remained consistently shallow, supporting interpretations of fluid-assisted fault slip within the seismogenic zone. No surface rupture or significant damage was associated with these events. Continued monitoring by regional seismic networks remains essential for understanding strain accumulation and potential links to nearby volcanic or geothermal systems.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey, Fault Activity Map of California
SCEC Community Fault Model documentation