Seismic Swarm S20211001.1 Near Mina, Nevada: Event Analysis and Regional Context

Seismic swarm S20211001.1 was recorded 34 km southeast of Mina, Nevada, beginning at 20:38 on 30 September 2021 and concluding at 11:07 on 7 October 2021. Over 158 hours and 28 minutes, the sequence produced 87 earthquakes. Magnitudes ranged from -0.1 to 2.5, with the largest event occurring on 1 October at a depth of 8 km. Depths throughout the swarm clustered between 3 km and 13 km, indicating shallow crustal activity typical of the Basin and Range extensional regime.

The temporal distribution showed peak activity on 1 October, when 24 events were registered, followed by a gradual decline. Early events on 30 September included magnitudes of 1.5, 0.7, and 2.2. Subsequent days featured repeated events near magnitude 2.0, with a notable 2.3 on 3 October and a final 1.4 on 7 October. The absence of a single dominant mainshock and the clustered occurrence of similar-sized events align with swarm characteristics rather than a classic foreshock-aftershock sequence.

The Mina region lies within the Walker Lane belt, a northwest-trending zone of right-lateral shear and extension that accommodates a significant portion of Pacific-North America relative plate motion. This tectonic setting produces frequent normal and strike-slip faulting on Quaternary-active structures. The local geology comprises Miocene volcanic rocks, basin-fill sediments, and fault-bounded ranges shaped by ongoing extension since the late Cenozoic. Historical seismicity in Mineral County reflects this framework, with documented earthquake swarms linked to fluid migration and stress transfer along segmented faults.

Since 1 January 2000, 21 swarms have occurred in the vicinity. Earlier episodes were recorded in 2003 (two swarms), 2013 (two swarms), 2020 (twelve swarms), and 2021 (five swarms). These recurrent swarms suggest persistent low-level strain accumulation and episodic release, consistent with the diffuse deformation across the Walker Lane.

Analysis of S20211001.1 indicates a compact spatial footprint and limited maximum magnitude, implying modest energy release and low likelihood of damaging ground motion. Continued monitoring remains essential given the region’s tectonic maturity and history of swarm activity.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
Nevada Seismological Laboratory, University of Nevada, Reno
Walker Lane Tectonic Framework, Nevada Bureau of Mines and Geology