Seismic Swarm S20200305.2: Analysis of Activity Southwest of Olancha, California

A seismic swarm designated S20200305.2 occurred 18 km southwest of Olancha, California, beginning at 12:50 UTC on 5 March 2020 and concluding at 18:37 UTC on 6 March 2020. Over 29 hours and 47 minutes, the sequence produced 42 earthquakes, all of low magnitude. The events clustered at shallow depths between 2 km and 9 km, with the majority occurring in the first several hours.

The initial events registered magnitudes of 1.3 and 1.0, followed rapidly by additional shocks reaching 1.7, 1.9, 2.1, and 2.2 within the first two hours. Subsequent activity included repeated events near magnitude 1.0–1.8, with notable repetitions at 18:27 and 21:47 on 5 March. Depths remained consistently shallow, indicating brittle failure within the upper crust. The swarm exhibited typical characteristics of clustered microseismicity, with no single dominant mainshock.

This sequence fits within the broader seismic regime of the region. The area southwest of Olancha lies in the Basin and Range Province, where extensional tectonics drive normal and strike-slip faulting along the Sierra Nevada frontal fault system and the Eastern California Shear Zone. The Coso Volcanic Field, located nearby, contributes geothermal influences and elevated background seismicity. Shallow crustal depths recorded in the swarm align with known fault structures in this transitional zone between the Sierra Nevada and the Owens Valley.

Historical records indicate recurrent swarm activity in the vicinity. Since 1 January 2000, 22 swarms have been documented in the area. Yearly counts include three each in 2000 and 2001, one in 2002, two in 2004, and single events in 2005, 2006, 2010, 2012, 2014, and 2015. The years 2013 and 2019 recorded two and five swarms, respectively. These episodes underscore persistent, episodic strain release along local fault networks.

The 2020 swarm’s magnitude distribution and temporal clustering resemble prior episodes, reinforcing the pattern of swarm-dominated rather than mainshock-aftershock sequences in this portion of eastern California. No damage or felt reports were associated with the small events.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey regional fault maps
SCEC Community Fault Model documentation