Seismic Swarm S20151018.1 Near Virginia City, Nevada

A minor earthquake swarm designated S20151018.1 was recorded 12 km northwest of Virginia City, Nevada, between 04:06 on 18 October 2015 and 10:38 on 19 October 2015. In approximately 30 hours and 31 minutes, 38 events were detected, all of very low magnitude and shallow focal depth. Magnitudes ranged from -0.3 to 1.7, with the majority falling below 1.0; depths were concentrated between 2 km and 4 km. The largest event, magnitude 1.7, occurred at 03:20 on 19 October at 4 km depth. Activity was tightly clustered in both space and time, consistent with swarm behavior rather than a mainshock-aftershock sequence.

Virginia City lies within the western Basin and Range Province, a tectonically active region undergoing east-west crustal extension. This extension is accommodated primarily by normal faulting along north- to northeast-striking range-front faults. The local geology comprises Tertiary volcanic rocks and Mesozoic basement intruded by granitic bodies, with the Comstock fault system and related structures providing pathways for fluid migration that can influence swarm occurrence. Historical seismicity in the area includes both tectonic events and possible mining-related activity dating back to the Comstock Lode silver boom that began in 1859.

Since 1 January 2000, 25 swarms have been documented in the same general region. Earlier episodes occurred in 2003, 2004, 2005, 2008 (seven swarms), 2010, 2012 (four), 2013 (four), 2014 (four), and two in 2015. The 2015 swarm fits the established pattern of short-duration, low-magnitude clusters at shallow depths, typical of fluid-driven seismicity in the Basin and Range.

The October 2015 sequence began with a rapid succession of events on 18 October, peaked in rate during the late morning and early afternoon, then declined sharply overnight before a final burst on 19 October. All recorded events remained below the threshold for felt shaking or damage, illustrating the low seismic hazard posed by such microswarms despite their frequency in this part of Nevada.

References

• Nevada Seismological Laboratory, University of Nevada, Reno – regional earthquake catalog and swarm statistics
• U.S. Geological Survey Earthquake Hazards Program – Basin and Range Province tectonic framework
• Western Mining History – Comstock Lode development timeline