Seismic Swarm Near Hawthorne, Nevada: Details of Event S20151218.1

The December 2015 earthquake swarm designated S20151218.1 occurred approximately 19 km east-northeast of Hawthorne, Nevada. Activity began at 18:24 on 17 December 2015 and concluded at 12:30 on 18 December 2015, spanning 18 hours and 6 minutes during which 29 earthquakes were recorded.

Magnitudes in the swarm ranged from -0.3 to 1.9, with focal depths between 6 km and 13 km. The sequence featured an initial cluster of events on the evening of 17 December, including a magnitude 1.7 earthquake at 20:09 and another at 20:12. The largest event, magnitude 1.9, occurred at 20:34 on 17 December at a depth of 7 km. Subsequent activity included multiple events near magnitude 0.5–0.9 through the night and into the morning of 18 December, with the final recorded shock of magnitude 0.9 at 12:30 on 18 December at 7 km depth. Depths remained relatively shallow throughout, consistent with upper-crustal faulting.

Hawthorne lies within the Basin and Range Province of western Nevada, an area defined by active extensional tectonics driven by normal faulting. The local geology features north-northwest-trending mountain ranges separated by alluvial valleys, underlain by Paleozoic to Mesozoic sedimentary and volcanic rocks intruded by Cenozoic granitic bodies. The region forms part of the Walker Lane belt, a zone of dextral shear that accommodates a portion of Pacific–North American plate motion and hosts elevated seismicity.

Seismic swarms are recurrent in this setting, often linked to fluid migration along pre-existing faults rather than mainshock–aftershock sequences. Historical records maintained by SeismoSight indicate seven swarms in the Hawthorne area since 1 January 2000, occurring in 2011 (one swarm), 2012 (one swarm), 2014 (three swarms), and 2015 (two swarms). These episodes typically involve low-magnitude events at depths of 5–15 km and last from several hours to a few days.

The 2015 swarm fits established patterns of short-duration, low-magnitude activity within the same tectonic framework. No damage or felt reports were associated with these events, reflecting their small sizes and moderate depths.

References

SeismoSight internal swarm catalog (S20151218.1 parameters and historical statistics).
USGS Earthquake Hazards Program regional tectonic summaries for the Walker Lane belt.
Nevada Bureau of Mines and Geology geologic maps of Mineral County.