Seismic Swarm S20211005.2: Analysis of Activity South of Goldfield, Nevada

Seismic swarm S20211005.2 was recorded 41 km south of Goldfield, Nevada, beginning at 04:05 on 4 October 2021 and concluding at 14:55 on 8 October 2021. The sequence lasted 106 hours and 49 minutes and included 69 earthquakes. All events remained below magnitude 3.0, consistent with typical low-to-moderate swarm behavior in the region.

Magnitudes ranged from 0.2 to 2.9. The largest event, magnitude 2.9, occurred at 03:07 on 5 October at a depth of 5 km. Several other events reached magnitude 2.3–2.8, clustered primarily on 4 and 5 October. Depths were predominantly shallow, between 0 and 12 km, with occasional events extending to 16 km. Activity showed clear temporal clustering, with peak rates on the first two days followed by a gradual decline through 8 October.

The swarm occurred within the Basin and Range Province, where active crustal extension along normal faults produces frequent small-magnitude seismicity. The Goldfield area lies near the transition between the central Nevada seismic belt and the Walker Lane shear zone, a region known for distributed faulting and episodic swarm sequences. Historical records indicate 13 prior swarms in the same locale since 2000, occurring in 2000, 2001, 2002, 2006, 2007 (two events), 2010, 2011 (two events), 2012, 2015 (two events), and 2017. This pattern suggests recurring stress release along local fault networks without progression to a mainshock–aftershock sequence.

Such swarms commonly reflect fluid migration or aseismic slip triggering brittle failure at shallow depths. No damage or felt reports were associated with S20211005.2, aligning with the modest energy release observed. Continued monitoring remains important given the region’s tectonic setting and documented history of swarm recurrence.

References

SeismoSight internal swarm classification S20211005.2
U.S. Geological Survey Earthquake Hazards Program
Nevada Seismological Laboratory, University of Nevada, Reno