Seismic Swarm S20200515.1: Analysis of Activity Near Mina, Nevada

Seismic swarm S20200515.1 occurred 36 km southeast of Mina, Nevada, beginning at 11:03 on 15 May 2020 and concluding at 12:36 on 6 March 2021. Over 7081 hours and 32 minutes, the swarm registered 21,711 earthquakes. This prolonged sequence represents a significant episode of clustered seismicity in western Nevada.

The initial event reached magnitude 6.5 at a depth of 2 km, marking the largest shock. Subsequent events included multiple magnitudes above 4.0 within the first hours, such as 4.9 at 3 km depth and 5.1 at 6 km depth. Depths throughout the first 100 recorded events ranged from 0 to 19 km, with the majority concentrated between 3 and 10 km, indicating shallow crustal activity consistent with regional fault systems.

Analysis of the first 100 events reveals a rapid onset followed by decaying frequency and magnitude. Early shocks showed a mix of magnitudes from 1.9 to 6.5, predominantly in the 2.5–4.0 range after the mainshock. Shallow depths dominated, with many events at 5–8 km, suggesting rupture along near-surface structures. Event clustering occurred in bursts, with several magnitude 3+ events spaced minutes apart initially, transitioning to more dispersed activity over hours.

Western Nevada lies within the Basin and Range Province, where extensional tectonics drive normal faulting and frequent seismic episodes. The Mina region specifically occupies a transitional zone influenced by the Walker Lane shear belt, which accommodates dextral motion between the Pacific and North American plates. This setting produces distributed fault networks prone to swarm-like sequences rather than isolated mainshock-aftershock patterns.

Historical records since 2000 indicate four swarms in the area, with prior episodes in 2003 (two swarms) and 2013 (two swarms). These earlier events underscore recurrent seismic unrest tied to the same fault network, though the 2020–2021 swarm exhibited greater duration and event count.

The swarm's characteristics align with fluid migration or aseismic slip triggering mechanisms common in the region. Shallow focal depths support interpretations of activity within the upper crust, where brittle failure predominates. No surface rupture was associated, typical for moderate-magnitude sequences in this tectonic environment.

Continued monitoring remains essential given the area's history of episodic swarms. Such data contribute to refined seismic hazard assessments for nearby communities and infrastructure.

References

United States Geological Survey Earthquake Catalog
Nevada Seismological Laboratory Regional Reports
Basin and Range Province Tectonic Summaries (USGS Professional Papers)