Seismic Analysis of Swarm S20180801.1 in Western Nevada

An earthquake swarm designated S20180801.1 was recorded 9 km northeast of Spanish Springs, Nevada. The sequence began at 10:11 UTC on 1 August 2018 and concluded at 17:22 UTC on 3 August 2018, encompassing 38 events over 55 hours and 11 minutes. All recorded magnitudes remained below 1.5, with the largest reaching 1.4 on 2 August. Hypocentral depths clustered between 7 km and 12 km, indicating a shallow crustal source consistent with regional faulting patterns.

The swarm exhibited a typical pattern of low-magnitude events interspersed with brief lulls. Initial activity on 1 August featured repeated micro-earthquakes near 0.0–0.9 magnitude at depths of 10–11 km. Later that day, slightly larger events up to 1.3 occurred at shallower depths around 7 km. Activity on 2 August included an isolated 1.4 magnitude shock at 7 km depth, followed by sparse smaller events. The final event on 3 August registered 0.2 magnitude at 7 km. Such temporal clustering without a dominant mainshock distinguishes swarms from typical aftershock sequences.

Western Nevada lies within the Basin and Range Province, where ongoing crustal extension produces normal faulting along north-trending structures. Spanish Springs occupies a position near the northern Walker Lane, a zone of distributed shear that accommodates part of the Pacific–North America plate motion. This tectonic setting has produced recurrent seismic swarms since at least the early 2000s. Historical records indicate nine prior swarms in the immediate area since 1 January 2000, occurring in 2012 (one swarm), 2013 (four swarms), 2014 (three swarms), and 2016 (one swarm). These episodes reflect episodic fluid migration or aseismic slip along favorably oriented faults rather than steady tectonic loading.

Seismic swarms in this region generally pose low hazard to infrastructure because of their modest energy release. Nevertheless, they provide valuable data on fault connectivity and stress transfer within the local network. Depths consistently near 10 km align with the brittle–ductile transition zone mapped in regional velocity models, where small stress perturbations can trigger prolonged microseismicity.

Continued monitoring by regional seismic networks remains essential for distinguishing swarm behavior from foreshock sequences that might precede larger events. The 2018 swarm reinforces the established pattern of clustered, low-magnitude activity that characterizes the Spanish Springs area.

References
Nevada Seismological Laboratory regional catalog (2018 event parameters).
USGS Earthquake Hazards Program tectonic framework reports for the Walker Lane.
Published geological maps of the Reno–Spanish Springs 1:100,000 quadrangle.