Seismic Swarm S20191218.1: Analysis of Activity Near Rachel, Nevada

A minor earthquake swarm designated S20191218.1 occurred 51 km south-southwest of Rachel, Nevada, between 16:46 UTC on 18 December 2019 and 09:49 UTC on 19 December 2019. Over 17 hours and 2 minutes, the sequence comprised 48 events. Magnitudes ranged from -0.4 to 1.3, with the majority falling between -0.3 and 0.7; depths were consistently shallow, between 5 km and 7 km. The largest event reached magnitude 1.3 at 22:54 UTC on 18 December.

The swarm exhibited typical characteristics of low-magnitude, closely spaced activity clustered within a small volume. Early events were predominantly below magnitude 0.0, followed by a gradual increase that peaked with the two magnitude-1.1 shocks at 21:56 UTC on 18 December and 03:50 UTC on 19 December. Activity then declined steadily until termination. Depths remained stable, indicating a compact source zone without significant migration.

This region lies within the Basin and Range Province of southern Nevada, where crustal extension along north-south trending normal faults produces frequent small earthquakes. The area is situated near the southern margin of the Nevada National Security Site, a tectonically active zone influenced by ongoing Basin and Range deformation. Historical records indicate sparse swarm activity since 2000, with only two prior episodes documented in 2012 and 2015. The 2019 swarm represents the third such sequence in nearly two decades, underscoring the episodic nature of microseismicity in this portion of the province.

Geological context is dominated by Quaternary fault scarps and volcanic rocks associated with the nearby Death Valley–Pancake Range belt. Shallow focal depths of 5–7 km align with the brittle upper crust typical of extensional regimes in the Great Basin. No surface rupture or felt reports were associated with the sequence, consistent with the low magnitudes observed.

SeismoSight internal classification identifies the sequence as a distinct swarm without linkage to larger tectonic events. Continued monitoring remains important for understanding background seismicity patterns in this remote portion of Nevada.

References

USGS Earthquake Catalog
Nevada Seismological Laboratory regional reports
Basin and Range Province tectonic summaries (USGS)