Seismic Swarm S20210117.1 Near Olancha, California

On January 17, 2021, a seismic swarm designated S20210117.1 was recorded 13 km southwest of Olancha, California. The sequence began at 02:49 UTC and concluded at 11:52 UTC, encompassing a total of 25 earthquakes over nine hours and three minutes. Magnitudes ranged from 1.0 to 4.0, with the majority occurring at shallow depths between 0 and 3 km. The largest event reached magnitude 4.0 at 02:49:08, followed by a magnitude 3.7 at 03:14:49 and a magnitude 3.3 at 03:30:32. Subsequent activity consisted primarily of smaller events below magnitude 2.5, tapering off by late morning.

This swarm occurred within the tectonically active Owens Valley region of eastern California. The area lies along the boundary between the Sierra Nevada and the Basin and Range Province, where northwest-trending strike-slip faults and north-south normal faults accommodate dextral shear associated with Pacific–North American plate motion. The Owens Valley Fault Zone and nearby structures in the Eastern California Shear Zone contribute to elevated seismicity. Shallow focal depths observed in the swarm are consistent with brittle failure in the upper crust, typical of this extensional regime.

Seismic swarms have been a recurring feature in the Olancha vicinity. Since January 1, 2000, 44 swarms have been documented in the region. Activity was distributed across multiple years, with notable concentrations in 2000 (6 swarms), 2004 (7), 2019 (7), and 2020 (6). Earlier episodes occurred in 2001, 2002, 2005–2006, 2010, 2012–2017, with varying swarm counts per year. These clusters often reflect localized stress adjustments or fluid migration along pre-existing faults rather than mainshock–aftershock sequences driven by a single large rupture.

The January 2021 swarm fits within this established pattern of episodic, low-to-moderate magnitude activity. No damage or injuries were reported, consistent with the modest energy release and rural setting. Continued monitoring of such sequences provides valuable data on fault behavior and crustal dynamics in one of California’s most geologically dynamic zones.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey, Regional Fault Maps
SCEC Community Fault Model Documentation