Seismic Swarm S20190704.1: The 2019–2020 Searles Valley Sequence

The seismic swarm designated S20190704.1 began at 16:07 UTC on 4 July 2019 and concluded at 04:09 UTC on 11 February 2020. Centered 11 km southwest of Searles Valley, California, the sequence produced 22,493 earthquakes over 5,316 hours and 2 minutes. This prolonged episode occurred within the Eastern California Shear Zone, a tectonically active region characterized by distributed right-lateral strike-slip faulting that accommodates a portion of the Pacific–North America plate motion.

Searles Valley lies in the Mojave Desert portion of Kern County, near the intersection of the Garlock Fault and northwest-trending faults of the Walker Lane belt. The local geology consists of Quaternary alluvial fans and playa deposits overlying Mesozoic basement rocks fractured by numerous northwest- and northeast-striking faults. Regional strain rates derived from GPS measurements indicate 8–12 mm yr⁻¹ of dextral shear across the zone, consistent with the observed seismicity.

The first 100 events reveal a classic swarm pattern: an initial low-magnitude foreshock sequence followed by rapid escalation. Within 86 minutes of onset, a magnitude 6.4 mainshock occurred at approximately 10 km depth. Subsequent events in the first hours included multiple magnitude 3.5–4.4 shocks at depths ranging from 1 km to 15 km, indicating brittle failure across a vertically extensive fault network. Magnitudes declined gradually after the initial peak, with the majority of early events between 2.0 and 3.5.

Since 1 January 2000, only two prior swarms have been recorded in the immediate area: one in 2013 and one in 2015. Both were far smaller in event count and maximum magnitude, underscoring the exceptional scale of the 2019–2020 sequence.

The swarm’s duration and productivity reflect fluid-driven or aseismic-slip triggering mechanisms common in the region. Aftershock decay followed a modified Omori law, with activity persisting for more than seven months before returning to background levels. No surface rupture was documented, consistent with the moderate mainshock magnitude and distributed faulting style.

This episode highlights the capacity of the Eastern California Shear Zone to host long-lived seismic swarms capable of producing significant ground shaking without a single dominant mainshock. Continued geodetic and seismological monitoring remains essential for assessing evolving hazard in this structurally complex area.

References

USGS Earthquake Catalog (ComCat)
Southern California Seismic Network (SCSN) annual bulletins
USGS Quaternary Fault and Fold Database of the United States