Seismic Swarm S20191216.1: Analysis of Activity Near Searles Valley, California

Earthquake swarm S20191216.1 was recorded in the Searles Valley region of California, beginning at 18:42 on 15 December 2019 and concluding at 06:55 on 22 December 2019. Over this 156-hour period, a total of 188 earthquakes were detected, with the epicenter located 18 km south-southwest of Searles Valley.

The sequence reflects typical swarm behavior, featuring numerous small-magnitude events without a single dominant mainshock. Analysis of the first 100 events reveals magnitudes predominantly between 0.0 and 2.0, with occasional peaks reaching 3.6. Depths clustered mainly between 4 km and 9 km, though a few events extended to 13 km. Early activity on 15 December included low-magnitude tremors at depths of 4–7 km. Subsequent days showed gradual increases in both frequency and magnitude, with notable events such as a 3.4 on 16 December at 6 km depth and a 3.6 on 17 December at 7 km depth. Depths remained relatively consistent, indicating a shallow crustal source zone.

This swarm occurred within the Eastern California Shear Zone, a network of strike-slip faults accommodating tectonic strain between the Pacific and North American plates. The Searles Valley area features Quaternary sediments overlying basement rocks, with regional fault systems contributing to distributed seismicity. Historical records since 2000 indicate 18 prior swarms in the vicinity, underscoring recurrent seismic episodes. Notably, 2019 alone accounted for 16 such swarms, marking elevated activity following the July Ridgecrest sequence.

Such swarms may arise from localized stress perturbations or fluid involvement in fault zones, though precise mechanisms require further geophysical study. The 2019 concentration suggests possible afterslip or triggered responses in the broader shear zone.

References

SeismoSight internal swarm classification data
USGS Earthquake Catalog (regional tectonics background)