Seismic Swarm S20191214.1: Analysis of Activity Near Searles Valley, California

Seismic swarm S20191214.1 occurred 11 km south of Searles Valley in California’s Mojave Desert. The sequence began at 06:22 on 13 December 2019 and concluded at 05:01 on 14 December 2019, spanning 22 hours and 38 minutes. During this interval, 27 earthquakes were recorded, all of low magnitude.

Magnitudes ranged from 0.0 to 2.2, with the largest event (M2.2) occurring at 15:23 on 13 December at a depth of 3 km. Most events clustered between 2 km and 9 km depth, consistent with shallow crustal faulting typical of the region. The swarm exhibited a rapid onset followed by a gradual decline in activity, a pattern common in fluid-driven or stress-triggered sequences within the Eastern California Shear Zone.

Searles Valley lies in the Basin and Range province, where northwest-directed dextral shear interacts with the left-lateral Garlock Fault system. This tectonic setting produces distributed deformation across numerous small faults, favoring swarm-like behavior rather than large mainshock-aftershock sequences. The area experienced elevated seismicity following the July 2019 Ridgecrest earthquakes, whose aftershock zone extended into adjacent valleys.

Historical records indicate 17 swarms in the region since 2000, with a marked increase in 2019 (15 events). Earlier swarms occurred in 2013 and 2015. The concentration of activity in late 2019 aligns with post-Ridgecrest stress redistribution across the shear zone.

The shallow focal depths and low magnitudes observed in S20191214.1 suggest activation of secondary faults without significant rupture propagation. Such swarms contribute to long-term strain release and help delineate active fault networks in this tectonically active corridor.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey, Regional Geologic Maps
USGS Open-File Report on the 2019 Ridgecrest Sequence