Seismic Swarm S20170721.1: Analysis of Activity near Hawthorne, Nevada

An earthquake swarm designated S20170721.1 occurred 34 km south-southwest of Hawthorne, Nevada, in the western Great Basin. The sequence began at 06:49 UTC on 21 July 2017 and concluded at 05:08 UTC on 18 August 2017, spanning 670 hours and 18 minutes. During this period, 279 earthquakes were recorded, with the majority exhibiting low magnitudes and focal depths between 6 and 16 km.

The swarm unfolded within the Basin and Range province, a region characterized by active extensional tectonics and normal faulting. Hawthorne lies near the Walker Lane shear zone, where northwest-trending dextral strike-slip faults interact with north-south normal faults. This tectonic setting produces frequent small-magnitude seismicity, often clustered in swarms rather than mainshock-aftershock sequences. Depths observed in the swarm align with typical crustal seismicity in the area, reflecting brittle failure in the upper 15–20 km of the lithosphere.

Examination of the first 100 events reveals a rapid onset followed by sustained low-level activity. The initial event registered magnitude 2.7 at 8 km depth. Subsequent shocks ranged from negative magnitudes to 2.5, with most falling below 1.5. Depths remained concentrated between 7 and 11 km during the first 48 hours, indicating a compact source volume. Temporal distribution showed clusters of events separated by quiet intervals, a pattern consistent with fluid-driven triggering or aseismic slip on nearby faults.

Regional historical data indicate recurrent swarm activity. Since 1 January 2000, 18 swarms have been documented in the vicinity, distributed across specific years as follows: 2001 (1 swarm), 2006 (2), 2011 (7), 2013 (1), 2016 (3), and 2017 (4). These episodes suggest episodic release of strain along pre-existing structures, possibly modulated by groundwater movement or minor magmatic influence at depth, though no surface volcanism is present.

The 2017 swarm fits this established pattern without exceeding typical energy release for the locale. No damage or felt reports of significance were associated with the sequence, underscoring the low hazard posed by such events despite their frequency.

References

SeismoSight internal swarm classification database
USGS Earthquake Catalog (regional tectonics summary)
Nevada Bureau of Mines and Geology (Basin and Range seismic history)