Seismic Swarm S20150918.1 near Hawthorne, Nevada: Characteristics and Regional Context

The earthquake swarm designated S20150918.1 occurred approximately 23 km east-southeast of Hawthorne, Nevada, within the tectonically active Basin and Range province. It began at 00:20 UTC on 18 September 2015 and concluded at 05:56 UTC on 23 September 2015, spanning 125 hours and 35 minutes. During this interval, 88 earthquakes were recorded, with the majority exhibiting magnitudes below 1.0 and focal depths predominantly between 0 and 5 km.

A notable event of magnitude 3.6 occurred at 20:11 UTC on 18 September at a depth of 5 km, representing the largest shock in the sequence. Subsequent activity consisted of numerous microearthquakes, including events of magnitude 1.9 and 1.4, clustered within the first 48 hours. Depths remained shallow throughout, with only isolated events reaching 10–11 km later in the swarm. This pattern aligns with typical swarm behavior in the region, where fluid migration or aseismic slip along normal faults can trigger prolonged sequences of small events without a dominant mainshock.

The Hawthorne area lies within the Walker Lane belt, a northwest-trending zone of dextral shear and extension that accommodates roughly 20 percent of Pacific–North America relative plate motion. Active normal and strike-slip faults dissect the Excelsior Mountains and adjacent basins, producing shallow crustal seismicity. Historical records since 2000 document eight prior swarms in the immediate vicinity, occurring in 2006, 2011, 2012, 2014 (four episodes), and an earlier 2015 sequence. These recurrent swarms reflect ongoing tectonic strain accumulation and release along distributed fault networks.

Seismic swarms in this portion of western Nevada commonly last from several days to weeks and rarely produce damaging ground shaking. The 2015 sequence remained well below thresholds for felt effects beyond the immediate epicentral zone. Continued monitoring by regional networks is essential for distinguishing swarm activity from foreshock sequences that might precede larger events.

References

Nevada Seismological Laboratory, University of Nevada, Reno.
U.S. Geological Survey Earthquake Hazards Program.
Western Mining History – Mineral County, Nevada geology overview.