Seismic Swarm S20190906.1 Near Olancha, California: Characteristics and Regional Context
A seismic swarm designated S20190906.1 occurred 16 km west-southwest of Olancha, California, between 06:49 on 5 September 2019 and 11:27 on 6 September 2019. Over 28 hours and 37 minutes, the sequence produced 27 earthquakes. All events registered magnitudes between 0.8 and 2.3 and originated at shallow depths of 1–4 km.
The swarm began with a magnitude-1.8 event at 4 km depth. Subsequent activity remained low-magnitude and clustered in time, with the largest shock (magnitude 2.3) occurring at 17:44 on 5 September at 2 km depth. Events tapered off overnight, concluding with a magnitude-1.0 shock at 2 km depth on 6 September. Depths were consistently shallow, indicating brittle failure within the uppermost crust.
This sequence fits the pattern of swarm activity documented in the area since 2000. Eighteen swarms have been recorded in the region, distributed across the years 2000 (3), 2001 (1), 2002 (1), 2004 (2), 2005 (1), 2006 (1), 2012 (1), 2013 (2), 2014 (1), 2015 (1), and 2019 (4). Such recurrent swarms reflect ongoing strain release without a single dominant mainshock.
Geologically, the swarm epicenter lies within Owens Valley, part of the Eastern California Shear Zone. This zone accommodates a portion of Pacific–North American plate motion through distributed right-lateral shear and normal faulting. The valley is bounded by the Sierra Nevada frontal fault system to the west and the Inyo Mountains fault to the east. Quaternary sediments and volcanic rocks overlie Mesozoic basement, creating a structural setting conducive to shallow seismicity. Historical large-magnitude earthquakes, such as the 1872 Owens Valley event, demonstrate the region’s capacity for significant strain release, although most contemporary activity occurs as small swarms or aftershock sequences.
The shallow focal depths observed in S20190906.1 are consistent with the high heat flow and fractured upper crust typical of the Basin and Range province transition zone. Fluid migration along pre-existing faults likely contributed to the diffuse, non-mainshock character of the swarm.
References:
USGS Earthquake Catalog (events 2000–2019)
California Geological Survey, Owens Valley fault compilation
USGS Quaternary Fault and Fold Database of the United States