Seismic Swarm Near Hawthorne, Nevada: The September 2015 Event

A seismic swarm designated S20150911.1 occurred 12 km east of Hawthorne, Nevada, from 03:06 on 11 September 2015 to 23:43 on 13 September 2015. Over 68 hours and 37 minutes, the sequence produced 39 earthquakes. Magnitudes ranged from -0.3 to 2.5, with the majority between 0.0 and 1.5. Focal depths clustered between 5 and 10 km, indicating shallow crustal activity typical of the region.

The swarm began with a magnitude 1.6 event at 9 km depth. Subsequent activity included two magnitude 2.3 shocks on 11 September at depths of 7 km and 6 km, and a peak magnitude 2.5 event at 6 km depth later that afternoon. Smaller events, some with negative magnitudes, continued through 12 and 13 September, tapering to a final magnitude 0.1 shock at 10 km depth. The temporal pattern showed higher rates on the first day, followed by a gradual decline, consistent with swarm behavior driven by fluid migration or aseismic slip rather than a single mainshock-aftershock sequence.

This event fits into a longer record of swarm activity in the area. Since 2000, 14 swarms have been documented near Hawthorne. Prior episodes occurred in 2006 (1 swarm), 2011 (7 swarms), 2012 (1 swarm), 2013 (1 swarm), 2014 (3 swarms), and 2015 (1 swarm, the present sequence).

The Hawthorne region lies within the western Basin and Range province of Nevada, where crustal extension along normal faults produces frequent small earthquakes. The site is also influenced by the northern Walker Lane belt, a zone of distributed right-lateral shear that accommodates part of the Pacific-North America plate boundary motion. This tectonic setting promotes both isolated events and episodic swarm sequences at depths of 5–12 km, matching the observed focal depths.

Historical monitoring by regional networks confirms that such swarms are common in Mineral County and adjacent areas, often without producing damaging ground motion. The 2015 sequence remained well below thresholds that would trigger felt reports beyond the immediate vicinity.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
Nevada Seismological Laboratory, University of Nevada, Reno
Nevada Bureau of Mines and Geology, Geological Map of Nevada