Seismic Swarm S20191119.1: Analysis of Activity near Searles Valley, California

Seismic swarm S20191119.1 was recorded beginning at 01:11 on 18 November 2019 and concluding at 04:58 on 28 November 2019. The sequence was located 20 km west-northwest of Searles Valley, California, and comprised 223 earthquakes over a period of 243 hours and 47 minutes.

The events clustered in a region of the northern Mojave Desert characterized by distributed strike-slip faulting within the Eastern California Shear Zone. This tectonic domain accommodates a portion of the Pacific-North America plate motion through numerous northwest-trending faults. Searles Valley itself occupies a pull-apart basin bounded by the Garlock Fault to the south and segments of the Little Lake Fault zone to the north. Regional seismicity has been elevated since the July 2019 Ridgecrest earthquake sequence, whose mainshocks occurred approximately 50 km to the northeast and triggered widespread aftershock activity across adjacent fault networks.

Examination of the first 100 catalogued events reveals predominantly microseismic activity. Magnitudes ranged from 0.0 to 2.3, with the majority falling between 0.3 and 1.3. Depths were shallow, concentrated between 1 km and 12 km, consistent with brittle failure in the upper crust. The initial event on 18 November registered magnitude 0.8 at 6 km depth, followed by a gradual increase in event rate through the first 24 hours. Two magnitude-2.3 events occurred on 22 November at depths of 6 km and 1 km, representing the largest shocks within the examined subset. No clear migration pattern is evident in the early sequence, though events remained tightly grouped spatially.

Historical records indicate persistent swarm activity in the area. Since 1 January 2000, 76 swarms have been identified in the same locale. Annual counts show variability, with notable increases in 2004 (7 swarms), 2006 (6), 2010 (7), and especially 2019 (25). This uptick aligns with the post-Ridgecrest stress readjustment period, during which fluid migration and aseismic slip may have contributed to swarm triggering.

Overall, swarm S20191119.1 exemplifies the low-magnitude, high-frequency seismicity typical of the Searles Valley region following major nearby ruptures. Continued monitoring remains essential given the area's proximity to active fault structures.

References

SeismoSight internal swarm classification records.
USGS Earthquake Hazards Program regional tectonic summaries for the Mojave Desert.