Seismic Swarm S20210213.1 Near Olancha, California: Analysis and Regional Context

A notable earthquake swarm, designated S20210213.1, occurred 11 km south of Olancha, California, from 19:54 on 12 February 2021 to 13:34 on 14 February 2021. Over 41 hours and 39 minutes, the swarm produced 29 earthquakes, with magnitudes ranging from 0.6 to 4.0 and depths between 2 and 9 km.

The sequence began with a magnitude 1.1 event at 3 km depth. Activity intensified rapidly, culminating in the largest event—a magnitude 4.0 quake at 3 km depth at 23:57 on 12 February. Subsequent events included a magnitude 2.8 at 3 km depth and multiple magnitude 2+ shocks clustered in the first hours. The swarm featured a typical decay pattern, with smaller events (magnitudes below 2.0) dominating later stages and the final recorded shock at magnitude 0.9 and 5 km depth.

This swarm aligns with the broader seismic character of the region south of Olancha in Inyo County. The area forms part of the Eastern California Shear Zone, where distributed right-lateral strike-slip faulting accommodates Pacific-North American plate motion. Normal faults associated with Basin and Range extension also contribute to the tectonic setting. Depths recorded in the swarm are consistent with shallow crustal seismicity prevalent along these structures.

The Coso Volcanic Field, located nearby, influences local seismicity through geothermal fluid circulation and magmatic processes, frequently generating swarm-like sequences. Historical records show persistent swarm activity in this zone. Since 2000, 100 swarms have been documented in the vicinity, with notable annual totals including 24 in 2019, 15 in 2020, and earlier peaks such as 9 in 2004 and 7 in 2000.

The 1872 Owens Valley earthquake, a magnitude 7.4–7.6 event approximately 50 km north, underscores the region's capacity for larger, damaging quakes along major faults. Modern monitoring by the USGS and California Geological Survey confirms ongoing microseismicity and occasional swarms as characteristic features rather than precursors to major events.

Swarm statistics indicate variable yearly occurrence without a clear long-term trend, reflecting episodic stress release modulated by fluids and fault interactions. Depths predominantly under 5 km in the 2021 sequence further support a shallow, possibly fluid-driven mechanism common in the Coso-Olancha corridor.

References

• United States Geological Survey Earthquake Catalog (USGS)
• California Geological Survey Regional Fault Maps
• Southern California Earthquake Data Center Historical Records