Seismic Swarm S20190910.1: Activity 20 km West of Searles Valley, California
Seismic swarm S20190910.1 occurred in the Mojave Desert region of eastern California, centered approximately 20 km west of Searles Valley. The sequence began at 09:35 UTC on 10 September 2019 and concluded at 07:01 UTC on 14 September 2019, spanning 93 hours and 25 minutes. During this interval, 145 earthquakes were recorded.
Analysis of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from 0.1 to 2.9, with the majority below 1.5. The largest event reached magnitude 2.9 at a depth of 3 km on 10 September. Subsequent notable shocks included magnitudes 2.7 and 2.5, both at shallow depths near 3 km. Depths varied between 0 and 13 km, clustering most frequently between 2 and 8 km, consistent with brittle failure in the upper crust.
Temporal distribution showed peak rates during the first 24 hours, followed by a gradual decline. Early events on 10 September included several above magnitude 1.0 within the initial hours, transitioning to smaller events over subsequent days. Depths remained shallow throughout, indicating activity within the seismogenic zone typical of the local fault network.
The swarm unfolded in a tectonically active corridor of the Eastern California Shear Zone. This region accommodates right-lateral shear between the Pacific and North American plates through a network of northwest-trending strike-slip faults. Searles Valley lies near the intersection of these structures with the Garlock Fault, a major left-lateral feature that bounds the southern Sierra Nevada. Historical data since 2000 document 26 prior swarms in the immediate area, with notable clusters in 2010 (two events), 2013 (two), and especially 2019 (eighteen). The July 2019 Ridgecrest sequence, featuring a magnitude 7.1 mainshock only 3 km from the swarm centroid, underscores the elevated background seismicity.
Such swarms typically reflect fluid migration or aseismic slip triggering small events along distributed fractures rather than mainshock-aftershock sequences. Depths and magnitudes align with patterns observed in other Mojave Desert swarms, where activity rarely exceeds magnitude 3.0 in the absence of larger triggering events.
References
SeismoSight internal swarm catalog (S20190910.1 parameters and event list).
USGS Earthquake Catalog (regional fault framework and Ridgecrest sequence details).