Seismic Swarm S20211018.1: Analysis of Activity Near Olancha, California

The seismic swarm designated S20211018.1 occurred 11 km east-southeast of Olancha, California, in a tectonically active portion of the Owens Valley within the Eastern California Shear Zone. This region experiences distributed right-lateral strike-slip faulting and extension linked to the broader interaction between the Pacific and North American plates, as well as Basin and Range province dynamics. The Coso Volcanic Field lies nearby to the south, contributing to elevated geothermal gradients and episodic seismicity influenced by both tectonic and magmatic processes.

The swarm initiated at 13:19 on 17 October 2021 with a magnitude 3.9 earthquake at 4 km depth. Over the subsequent 31 hours and 29 minutes, 27 earthquakes were recorded before activity concluded at 20:48 on 18 October 2021. The sequence featured a mainshock followed by numerous aftershocks, with magnitudes ranging from 0.7 to 3.9 and focal depths predominantly between 4 km and 8 km. Notable events included a magnitude 2.6 quake at 21:41 on 17 October, a magnitude 2.5 event at 00:41 on 18 October, and several magnitude 2+ shocks clustered in the afternoon and evening of the first day. Shallower events (1–2 km) occurred later in the sequence, while the deepest reached 8 km.

This swarm reflects typical behavior in the Olancha area, where fluid migration along faults or minor magmatic intrusions can trigger clusters of events without a single dominant mainshock-aftershock pattern. The Owens Valley has a well-documented history of seismic swarms driven by these mechanisms, alongside larger tectonic earthquakes such as the 1872 Owens Valley event.

Historical records indicate 77 swarms in the region since 1 January 2000. Annual counts varied considerably, with peaks of nine swarms each in 2004 and 2019. Other notable years included eight swarms in 2020, six each in 2000 and 2006, and five each in 2010 and 2013. Lower activity years featured one swarm in 2011, 2015, and 2018. The 2021 total reached four swarms, consistent with ongoing monitoring of this seismically productive zone.

Such swarms provide valuable data for understanding fault interactions and stress transfer in the Eastern California Shear Zone. Continued observation supports improved hazard assessment for nearby communities and infrastructure.

References

SeismoSight internal swarm classification S20211018.1
USGS Earthquake Hazards Program regional tectonic summaries