Seismic Swarm S20190525.1 Near Olancha, California

A seismic swarm designated S20190525.1 was recorded 18 km west-southwest of Olancha, California. The sequence began at 23:39 UTC on 24 May 2019 and concluded at 12:27 UTC on 26 May 2019, encompassing 43 earthquakes over 36 hours and 47 minutes. All events were of low magnitude, with the largest reaching 2.1, and hypocentral depths remained shallow, ranging from 0 to 5 km.

The swarm exhibited classic characteristics of clustered microseismicity without a dominant mainshock. Events occurred in episodic bursts, particularly between 01:00 and 08:00 UTC on 25 May, when multiple shocks of magnitude 1.3–2.1 were registered within minutes of one another. Magnitudes were predominantly below 1.5, and the temporal distribution showed a gradual decline in frequency after the initial peak, consistent with swarm behavior driven by fluid migration or aseismic slip rather than static stress transfer from a single large rupture.

Olancha lies within Owens Valley in eastern California, a tectonically active region situated at the western margin of the Basin and Range province. The valley occupies a transtensional setting influenced by the Eastern California Shear Zone and the Sierra Nevada frontal fault system. Right-lateral strike-slip and normal faulting accommodate northwest-directed Pacific–North America plate motion. The area has produced notable historical earthquakes, including the 1872 Owens Valley event (estimated magnitude 7.4–7.6), which generated surface rupture along the Owens Valley fault and caused widespread damage. Smaller instrumented events occur regularly, reflecting ongoing distributed deformation.

Since 2000, eleven swarms have been documented in the immediate vicinity, occurring in 2000 (3 swarms), 2002 (1), 2004 (2), 2005 (1), 2006 (1), 2013 (2), and 2014 (1). These episodes typically involve dozens of events of magnitude less than 3 and share similar shallow focal depths, underscoring the persistent low-level seismic productivity of the local fault network.

The S20190525.1 sequence aligns with this regional pattern. Its brief duration, limited magnitude range, and spatial concentration indicate a localized stress perturbation rather than a harbinger of larger rupture. Continued monitoring of such swarms contributes to refined understanding of fault interactions and improves probabilistic seismic hazard assessments for the Owens Valley corridor.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey, Quaternary Fault and Fold Database
Southern California Earthquake Data Center, historical seismicity records