Seismic Swarm S20171021.1 Near Hawthorne, Nevada

A notable earthquake swarm designated S20171021.1 occurred approximately 26 km south-southwest of Hawthorne, Nevada, between 20:55 UTC on 20 October 2017 and 02:47 UTC on 23 October 2017. Over this 53-hour and 51-minute period, 61 earthquakes were recorded, with magnitudes ranging from 0.5 to 2.0 and focal depths between 3 km and 13 km. The sequence exhibited typical swarm characteristics, featuring a gradual onset, fluctuating event rates, and no single dominant mainshock. The events clustered tightly in both space and time, beginning with several magnitude-1.4 shocks on the evening of 20 October. Activity intensified overnight, including a magnitude-1.7 event at 01:14 UTC on 21 October, before tapering through the following days. The largest shock reached magnitude 2.0 at 10:22 UTC on 21 October. Depths remained predominantly shallow, consistent with brittle failure in the upper crust. Hawthorne lies within the western Basin and Range Province, specifically along the Walker Lane shear zone—a transitional belt accommodating northwest-directed dextral shear between the Sierra Nevada block and the extending Basin and Range interior. This tectonic setting produces abundant normal and strike-slip faults that host frequent small-magnitude seismicity. Regional geology comprises Miocene to Quaternary volcanic and sedimentary rocks overlying Mesozoic basement, with active fault systems such as the Wassuk Range fault zone and the Benton Spring fault contributing to the observed deformation. Nevada ranks among the most seismically active states in the contiguous United States. Instrumental records document numerous earthquake swarms in the Hawthorne area since 2000, totaling 20 such episodes through 2017. Earlier swarms occurred in 2001 (1 swarm), 2006 (2), 2011 (7), 2013 (1), 2016 (3), and 2017 (6), underscoring the recurrent nature of clustered seismicity in this portion of the Walker Lane. Such swarms typically arise from fluid migration or slow slip on faults rather than abrupt tectonic loading. Although individual events in S20171021.1 remained below damaging thresholds, the sequence illustrates the persistent background hazard in Mineral County and the value of continuous seismic monitoring for understanding strain accumulation along the Pacific–North America plate boundary.

References

USGS Earthquake Catalog
Nevada Seismological Laboratory regional reports
Walker Lane tectonic summaries (USGS Professional Papers)