Seismic Swarm Near Virginia City, Nevada: Analysis of Swarm S20240129.1

A seismic swarm designated S20240129.1 was recorded northwest of Virginia City, Nevada, between 01:06 on 28 January 2024 and 11:29 on 2 February 2024. The sequence lasted 130 hours and 22 minutes and comprised 90 earthquakes. The epicentral area lies 11 km NNW of Virginia City at shallow focal depths predominantly between 4 km and 9 km.

Event magnitudes remained low throughout the swarm. The largest shock reached magnitude 3.5 on 29 January at 00:03:00 UTC. Subsequent events clustered between magnitude 0.1 and 1.9, with the majority falling below magnitude 1.0. Depths showed minor variation, with a few outliers at 0 km and 4 km, but the bulk of activity concentrated near 7–8 km.

The swarm unfolded in distinct phases. Initial activity on 28 January consisted of scattered events below magnitude 1.5. A notable increase occurred after midnight on 29 January, culminating in the magnitude-3.5 event followed by dozens of smaller aftershocks over the next 24 hours. Activity continued at a reduced rate through 30–31 January and tapered off by early February, ending with a magnitude-1.1 event on 2 February.

Virginia City sits within the western Basin and Range Province, a region undergoing active crustal extension. The local geology is dominated by Miocene volcanic rocks of the Virginia Range overlying Mesozoic basement. Numerous normal and strike-slip faults accommodate regional strain associated with the northern Walker Lane belt. This tectonic setting produces frequent small-magnitude earthquake swarms rather than large mainshock-aftershock sequences.

Historical records maintained by SeismoSight document 36 swarms in the same source area since 1 January 2000. Yearly counts include single swarms in 2003, 2004, 2005, 2010, 2017, 2023 and 2024; seven swarms in 2008; four each in 2012–2015; and two each in 2016, 2018 and 2019. These recurrent swarms reflect persistent low-level strain release along the same fault network.

Such swarms are characteristic of the region and are commonly linked to fluid migration or aseismic slip on pre-existing faults. No damage or felt reports of significance accompanied the 2024 sequence, consistent with the modest magnitudes observed.

References

SeismoSight internal swarm catalog (S20240129.1 parameters and historical statistics).
USGS Earthquake Hazards Program – Nevada seismicity overview.
Nevada Bureau of Mines and Geology – Geologic map of the Virginia City quadrangle.