Seismic Swarm S20161124.1: Analysis of Activity Near Virginia City, Nevada

Seismic swarm S20161124.1 occurred approximately 14 km northwest of Virginia City, Nevada, within the Basin and Range extensional province. The sequence began at 21:29 on 23 November 2016 and concluded at 03:42 on 26 November 2016, spanning 54 hours and 12 minutes. During this interval, 34 earthquakes were recorded, with magnitudes ranging from -0.2 to 1.7 and focal depths primarily between 4 km and 10 km. The largest event, magnitude 1.7, initiated the swarm, followed by a dense cluster of smaller events concentrated in the first 24 hours.

The swarm exhibited typical characteristics of fluid-driven or stress-triggered sequences common in the region. Early activity on 23 November included multiple events near 7 km depth with magnitudes between 0.0 and 1.0. Subsequent days showed a gradual decline in both frequency and magnitude, with isolated events persisting until the final recorded shock of magnitude 0.3 at 6 km depth. Depths remained shallow throughout, consistent with brittle failure in the upper crust of this tectonically active area.

Virginia City lies within the western Nevada seismic belt, where normal faulting accommodates ongoing extension between the Sierra Nevada and the stable North American interior. The local geology features Quaternary alluvium overlying Tertiary volcanic rocks associated with the ancestral Cascade arc and Comstock Lode mineralization. Regional strain rates derived from GPS measurements indicate 1–2 mm per year of extension, sustaining recurrent microseismicity. Historical records document at least 30 swarms in the broader area since 2000, with notable clusters in 2008 (seven events) and multiple years recording three to four swarms each. These episodes reflect distributed faulting rather than mainshock-aftershock sequences on a single structure.

Such swarms contribute to long-term seismic hazard assessment by illuminating active fault segments and fluid pathways. Although individual events in S20161124.1 remained below felt thresholds, their cumulative pattern aligns with established tectonic frameworks for western Nevada. Continued monitoring supports refined models of crustal deformation in this portion of the Basin and Range.

References

• SeismoSight internal swarm classification records for S20161124.1
• USGS Earthquake Catalog (historical swarm statistics since 2000)
• Nevada Bureau of Mines and Geology regional tectonic summaries