Seismic Swarm S20190528.1: Analysis of Activity Near Olancha, California

A seismic swarm designated S20190528.1 occurred 19 km west-southwest of Olancha, California, beginning at 01:46 on 28 May 2019 and concluding at 05:28 on 30 May 2019. Over 51 hours and 42 minutes, the swarm produced 61 earthquakes. This sequence represents typical clustered seismicity in a tectonically active portion of eastern California.

The events exhibited magnitudes ranging from 0.8 to 2.6, with the largest shocks reaching 2.6 on 28 May at 19:54 and again on 29 May at 05:19. Depths remained shallow throughout, concentrated between 0 and 5 km. Early activity on 28 May included multiple events near magnitude 1.5–1.9 at depths of 2–3 km. Peak rates occurred during afternoon and evening hours on 28 May, featuring several magnitude-2+ shocks at very shallow depths of 0–1 km. Activity continued at a steady but lower rate through 29 May before tapering off by the morning of 30 May.

Seismic swarms in this region have been documented consistently since 2000. Records indicate 13 swarms in total during this period, occurring in the years 2000 (3 events), 2001 (1), 2002 (1), 2004 (2), 2005 (1), 2006 (1), 2013 (2), 2014 (1), and 2019 (1). The 2019 swarm aligns with this established pattern of episodic, low-magnitude clustering rather than a single large mainshock-aftershock sequence.

The Olancha area lies within the southern Owens Valley, part of the Eastern California Shear Zone and the broader Basin and Range Province. This setting features active normal and strike-slip faulting associated with ongoing extension and shear between the Sierra Nevada and the North American plate interior. Shallow seismicity is common due to the presence of fractured volcanic and sedimentary rocks overlying crystalline basement. The Coso geothermal field to the southeast further influences local stress conditions through fluid migration, which can trigger swarm-like behavior.

Historical large earthquakes underscore the region's tectonic vitality, including the 1872 Owens Valley event of estimated magnitude 7.4–7.9. Modern monitoring reveals that swarms like S20190528.1 release strain gradually without exceeding magnitude 3, reducing the immediate likelihood of a major rupture. Depths consistently under 5 km suggest activity occurs within the uppermost brittle crust, where fluid pressure or aseismic slip may facilitate repeated small failures.

Overall, the S20190528.1 swarm illustrates the persistent, distributed nature of seismicity along the western margin of the Great Basin. Continued observation of such sequences contributes to refined hazard assessments for nearby communities and infrastructure in Inyo County.