Analysis of Earthquake Swarm S20180112.1 Near Virginia City, Nevada

SeismoSight recorded swarm S20180112.1 beginning at 06:11 on 12 January 2018 and concluding at 09:41 on 20 January 2018. The sequence occurred 12 km northwest of Virginia City, Nevada, and comprised 354 earthquakes over 195 hours and 30 minutes.

The first 100 events displayed shallow focal depths predominantly between 4 km and 7 km. Magnitudes ranged from 0.1 to 2.6, with the largest event (M2.6) occurring at 10:06:39 on 12 January at 7 km depth. Most events clustered between M0.5 and M1.2, consistent with typical swarm behavior in which numerous small-magnitude shocks occur without a single dominant mainshock. Temporal distribution showed peak activity during the initial 24 hours, followed by a gradual decline.

Virginia City lies within the western Basin and Range Province, an extensional tectonic regime marked by normal faulting and distributed shear associated with the Walker Lane belt. This setting produces frequent earthquake swarms driven by fluid migration and crustal extension. The Comstock mining district, historically active since the 1850s, has long experienced induced and natural seismicity linked to these structures.

Historical data indicate 28 swarms have occurred in the region since 1 January 2000. Yearly counts include single swarms in 2003, 2004, 2005, 2010, and 2017; seven in 2008; three in 2012; five in 2013; two in 2014; four in 2015; and two in 2016. This recurrence underscores persistent seismic productivity northwest of Virginia City.

The 2018 swarm aligns with established patterns of low-to-moderate magnitude activity at shallow depths. No damage was reported, reflecting the modest energy release. Continued monitoring remains essential given the region’s tectonic framework and historical swarm frequency.

References
SeismoSight internal swarm catalog S20180112.1
USGS Earthquake Catalog (Nevada events 2000–2018)
Nevada Bureau of Mines and Geology, Geological Map of the Virginia City Quadrangle