Seismic Swarm S20210927.1 Near Mina, Nevada: Characteristics and Regional Context

A seismic swarm designated S20210927.1 occurred approximately 26 km south-southeast of Mina in Mineral County, Nevada. The sequence began at 05:37 UTC on 26 September 2021 and concluded at 05:57 UTC on 28 September 2021, spanning roughly 48 hours and 20 minutes. During this interval, 31 earthquakes were recorded, with magnitudes ranging from 0.1 to 2.8 and focal depths between 0 and 12 km. The largest event, magnitude 2.8, took place on 26 September at 11:15 UTC at a depth of 10 km. Most events clustered in the initial 24 hours, with depths predominantly between 4 and 10 km, consistent with shallow crustal activity in the area.

This swarm reflects the ongoing extensional tectonics that characterize western Nevada. The region lies within the Basin and Range province and the northern Walker Lane belt, where northwest-directed shear and east-west extension produce abundant normal and strike-slip faulting. These structures accommodate a portion of the relative motion between the Pacific and North American plates, resulting in frequent low-to-moderate seismicity. Historical records indicate that the Mina area has hosted multiple earthquake swarms since 2000, with a total of 20 documented episodes. Notable prior activity occurred in 2003 (two swarms), 2013 (two swarms), 2020 (twelve swarms), and earlier in 2021 (four swarms). Such sequences typically involve numerous small events without a single dominant mainshock, distinguishing them from typical aftershock sequences.

The 2021 swarm events exhibited a temporal pattern of higher frequency on the first day, followed by rapid decay, with only two events recorded on the final day. Depths remained shallow throughout, rarely exceeding 10 km, aligning with the brittle upper crust in this tectonically active zone. No damage or felt reports of significance were associated with the sequence, as is common for events below magnitude 3.0 in sparsely populated regions.

Seismic swarms in this part of Nevada are often linked to fluid migration or aseismic slip along pre-existing faults within the Walker Lane system. Continued monitoring by regional networks helps refine understanding of strain accumulation and potential for larger events along nearby structures such as the Benton Spring Fault.

References

USGS Earthquake Catalog
Nevada Seismological Laboratory, University of Nevada, Reno
Nevada Bureau of Mines and Geology, Geological Overview of Mineral County