Seismic Swarm S20160927.1 Near Hawthorne, Nevada

A notable seismic swarm designated S20160927.1 was recorded 22 km east-southeast of Hawthorne, Nevada. The sequence began at 06:47 on 26 September 2016 and concluded at 21:47 on 29 September 2016, spanning 87 hours and encompassing 44 earthquakes.

Magnitudes within the swarm ranged from -0.7 to 3.2, with the largest event occurring at 09:51 on 26 September at a depth of 9 km. Most events clustered between 2 km and 6 km depth, indicating shallow crustal activity typical of the region. Smaller events dominated the sequence, with 28 events registering below magnitude 0.5 and only one exceeding magnitude 3.0. The temporal distribution showed peak activity on the first day, followed by a gradual decline through 29 September.

This swarm fits into a broader pattern of seismic activity in the Hawthorne area. Historical records since 2000 document 14 swarms, with notable clusters in 2014 and 2015 (four each) and three in 2016. Earlier swarms occurred in 2006, 2011, and 2012. Such recurrent swarms suggest episodic stress release along local fault systems rather than isolated mainshock-aftershock sequences.

The Hawthorne region lies within the western Basin and Range province of Nevada, characterized by extensional tectonics driven by the interaction between the Pacific and North American plates. The area forms part of the Walker Lane belt, a zone of distributed shear and normal faulting that accommodates a significant portion of the relative plate motion. Quaternary faults in Mineral County, including segments of the Wassuk Range fault system, contribute to the elevated seismicity. Depths recorded in the swarm align with the brittle-ductile transition zone expected in this extensional setting, where normal faulting predominates.

Seismic swarms in this tectonic environment often reflect fluid migration or aseismic slip triggering brittle failure on closely spaced faults. The 2016 sequence exhibited no clear mainshock, consistent with swarm behavior observed in prior events in the same locale. Depths predominantly between 0 km and 9 km further support shallow crustal processes linked to regional extension rates of several millimeters per year.

Continued monitoring of such swarms provides valuable data on fault interactions and strain accumulation in the Walker Lane. The S20160927.1 event underscores the persistent seismic hazard in western Nevada, where low-magnitude sequences can precede or accompany larger tectonic events.

References

SeismoSight internal swarm classification records.
USGS Earthquake Hazards Program regional tectonic summaries for the Basin and Range province.