Seismic Swarm S20200323.1 Near Searles Valley, California

Seismic swarm S20200323.1 was recorded 21 km north of Searles Valley in California’s Mojave Desert. The sequence began at 05:53 on 23 March 2020 and concluded at 08:38 on 27 March 2020, spanning 98 hours and 44 minutes. During this interval, 52 earthquakes were detected.

The initial event reached magnitude 4.3 at a depth of 0 km. Subsequent activity consisted predominantly of microearthquakes below magnitude 2.0, with isolated events up to magnitude 2.4. Depths remained shallow throughout, ranging from 0 km to 4 km. The swarm exhibited a typical decay pattern: higher-frequency small events clustered in the first 48 hours, followed by sporadic lower-magnitude occurrences until termination.

The Searles Valley region occupies the western Mojave Desert within the Eastern California Shear Zone. This zone comprises a network of northwest-trending strike-slip faults that accommodate approximately 20–25 % of Pacific–North American plate boundary motion. Local geology features Quaternary alluvium and lacustrine deposits overlying crystalline basement rocks of Mesozoic age. Searles Lake, a now-dry playa immediately south of the swarm epicenters, records repeated cycles of evaporation and mineral precipitation that have produced commercially significant evaporite beds.

Tectonic strain in the area is released through both large mainshock–aftershock sequences and distributed earthquake swarms. The 2019 Ridgecrest sequence, located roughly 30 km to the northeast, highlighted the potential for multi-fault rupture in this setting. Post-2019 monitoring has shown continued elevated microseismicity, consistent with the elevated swarm count recorded in 2019 and 2020.

Historical records indicate 24 swarms in the immediate region since 1 January 2000. Annual counts remained low until 2019, when 12 swarms occurred, followed by three additional swarms in 2020, including S20200323.1. This temporal clustering aligns with increased strain release following the Ridgecrest events.

Analysis of the 52 events shows no single dominant fault plane; rather, the spatial distribution suggests activation of a diffuse network of shallow fractures. The prevalence of events at or near 0 km depth may reflect fluid migration or thermal effects within the upper crust, common in swarm sequences observed elsewhere in the shear zone.

Continued seismic monitoring remains essential for assessing whether future swarms evolve into larger ruptures. The combination of shallow seismicity and complex fault architecture underscores the need for high-resolution networks to capture precursory patterns in this portion of the Eastern California Shear Zone.

References
USGS Earthquake Catalog (2020–2024 updates)
California Geological Survey, Mojave Desert Regional Fault Map
SeismoSight internal swarm classification records