Seismic Swarm S20200309.1: Analysis of Activity Near Searles Valley, California
Seismic swarm S20200309.1 was recorded beginning at 07:12 on 8 March 2020 and concluding at 05:15 on 21 March 2020. The events occurred 23 km NNW of Searles Valley, California. Over a span of 310 hours and 3 minutes, a total of 242 earthquakes were registered.
Analysis of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from -0.3 to 2.3, with the large majority below 1.0 and only a single event exceeding magnitude 2. Depths varied between 2 km and 12 km, clustering most frequently around 6–8 km. Early events on 8 March showed a mix of shallow and mid-crustal depths, while subsequent activity maintained similar depth distributions without clear migration patterns. Temporal spacing indicated episodic bursts, particularly on 8–10 March and 12–13 March, separated by quieter intervals.
The Searles Valley region lies within the Mojave Desert portion of the Eastern California Shear Zone, a tectonically active belt accommodating right-lateral shear between the Pacific and North American plates. The area features numerous northwest-trending strike-slip faults and is influenced by the nearby Garlock Fault system. Seismicity here is driven by distributed deformation rather than a single master fault. The 2019 Ridgecrest earthquake sequence, located immediately to the north, significantly elevated regional strain and aftershock rates, contributing to the observed increase in swarm occurrences.
Historical records document 87 swarms in the area since 1 January 2000. Annual counts show variability, with notable elevations in 2019 (29 swarms) and 2020 (8 swarms), consistent with post-Ridgecrest elevated seismicity. Earlier years recorded fewer events, typically between one and seven per year.
This swarm exemplifies the background clustered seismicity characteristic of the shear zone, where fluid migration or aseismic slip may trigger such sequences without producing a dominant mainshock.
References
USGS Earthquake Hazards Program – Regional Tectonic Setting of the Mojave Desert
California Geological Survey – Fault Activity Map of California
SeismoSight internal swarm classification database