Seismic Analysis of Swarm S20191108.1 Near Searles Valley, California
SeismoSight registered Swarm S20191108.1 beginning at 08:34 on 7 November 2019 and concluding at 16:46 on 16 November 2019. The sequence occurred 16 km south-southwest of Searles Valley in California’s Mojave Desert and produced 234 earthquakes over 224 hours and 12 minutes.
The first 100 events displayed predominantly low magnitudes, with values ranging from -0.2 to a peak of 3.4. Depths clustered between 1 km and 14 km, indicating shallow crustal activity typical of the region. Early activity on 7 November included an initial -0.2 event at 7 km depth followed rapidly by events of 0.4, 1.8, and 2.8. The 3.4 magnitude shock at 12:38 occurred at 9 km depth and represented the strongest event in the initial sequence. Subsequent events remained below magnitude 3.0, with most falling between 0.5 and 2.2. Temporal spacing showed clusters of activity separated by quieter intervals, a pattern consistent with swarm behavior rather than mainshock-aftershock sequences.
Searles Valley occupies the western Mojave Desert within an area of distributed extensional and strike-slip faulting. The local geology features Quaternary alluvium overlying older sedimentary and volcanic units, with active faults accommodating Pacific-North America plate motion. Seismic swarms recur in this setting due to fluid migration and stress transfer along secondary structures. Historical records since 2000 document 74 swarms in the immediate area, with notable yearly totals including four in 2000, seven in 2004, seven in 2010, and a marked increase to 23 swarms in 2019.
The 2019 swarm fits within this established pattern of episodic, low-magnitude activity. Depths recorded during the first 100 events align with the brittle-ductile transition zone expected in the Mojave crust, where temperatures permit brittle failure above approximately 15 km. No damage or felt reports beyond instrumental detection were associated with this swarm.
Continued monitoring of such sequences provides valuable data on fault interaction and regional strain accumulation. The elevated swarm count observed in 2019 underscores the importance of dense seismic networks for tracking subtle changes in background seismicity.
References
USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey regional fault maps
SeismoSight internal swarm classification database