Seismic Swarm S20151020.1: Microearthquake Activity Near Hawthorne, Nevada

On 20 October 2015, a compact seismic swarm designated S20151020.1 was recorded 30 km south-southeast of Hawthorne, Nevada. The sequence began at 02:30 UTC and concluded at 03:59 UTC, encompassing 28 earthquakes within a 1-hour-29-minute window. Magnitudes ranged from −0.1 to 0.9, with focal depths concentrated between 8 km and 12 km. The events clustered tightly in both space and time, exhibiting the characteristic pattern of swarm behavior in which no single mainshock dominates.

Individual events displayed minor depth variations, with the majority nucleating at approximately 8 km. Peak magnitudes occurred at 02:44:54 (M 0.8) and 03:44:50 (M 0.9). Such low-energy releases are typical of fluid-assisted or aseismic-slip-driven swarms rather than classic mainshock-aftershock sequences.

Regional Geological Setting

The swarm epicenters lie within the western Basin and Range Province, specifically along the northern margin of the Mina deflection in the Walker Lane belt. This zone accommodates approximately 20 % of the Pacific–North America relative plate motion through distributed right-lateral shear and clockwise rotation of crustal blocks. Active normal and strike-slip faults transect the area, producing the characteristic basin-and-range topography. Crustal thickness averages 30–35 km, and heat flow is elevated, consistent with ongoing extension and possible magmatic underplating at depth.

Seismicity in Mineral County is frequent yet rarely destructive because most events occur at shallow to mid-crustal depths on small-displacement faults. The 2015 swarm’s depth range aligns with the brittle–ductile transition zone where fluids or aseismic creep can trigger episodic microearthquake clusters.

Historical Swarm Context

Instrumental records since 1 January 2000 document 16 swarms within the same 30 km radius of Hawthorne. Yearly counts are as follows: 2001 (1), 2006 (3), 2011 (7), 2012 (1), 2013 (1), 2014 (1), and 2015 (2). The 2011 episode remains the most prolific, while later years show lower but persistent swarm activity. This temporal distribution suggests a quasi-periodic stressing cycle modulated by regional shear and local hydrothermal processes.

Implications and Monitoring Value

Although individual events in S20151020.1 were too small to be felt, the swarm underscores the value of dense seismic networks for tracking subtle strain transients. Continued monitoring can help distinguish between purely tectonic swarms and those potentially linked to deeper fluid migration or geothermal activity. Updated focal-mechanism studies and InSAR observations would further constrain whether the 2015 sequence involved any measurable aseismic slip.

References
SeismoSight internal swarm catalog (S20151020.1 parameters)
United States Geological Survey Earthquake Hazards Program regional tectonics summaries
Nevada Seismological Laboratory Walker Lane strain analyses