Seismic Swarm S20120819.1: Geological Context and Analysis Near Hawthorne, Nevada

Seismic swarm S20120819.1 was recorded 17 km east of Hawthorne, Nevada, beginning at 23:40 UTC on 18 August 2012 and concluding at 08:25 UTC on 21 August 2012. Over this 56-hour-and-45-minute period, 45 earthquakes were detected. This sequence exemplifies the clustered microseismicity typical of the region’s extensional tectonic setting.

The events displayed magnitudes between -0.6 and 1.8, with the largest shock reaching 1.8 at a depth of 8 km shortly after onset. Depths ranged from surface level to 12 km, though most hypocenters clustered between 4 km and 9 km. Activity initiated with several events near magnitude 1.7–1.8 within the first hour, followed by a prolonged tail of smaller tremors, many registering below magnitude 0.0. Notable later shocks included a magnitude 1.4 event on 20 August at 9 km depth and scattered magnitude 1.1–1.2 occurrences through 20 August. The temporal distribution showed peak frequency in the initial 12 hours, with diminishing rates thereafter.

Hawthorne lies within the Basin and Range province of western Nevada, specifically along the Walker Lane belt—a northwest-trending zone of right-lateral shear that accommodates approximately 20 percent of Pacific–North American plate motion. This belt features active normal and strike-slip faults that produce both swarm activity and occasional larger mainshock-aftershock sequences. The local geology comprises Quaternary alluvium overlying Miocene volcanic and sedimentary rocks, with underlying Paleozoic basement units dissected by range-bounding faults. Crustal extension rates in the area average 1–2 mm per year, contributing to the frequent occurrence of shallow earthquake swarms.

Historical records maintained by SeismoSight indicate nine swarms in the Hawthorne vicinity since 1 January 2000. Earlier episodes occurred in 2006 (two swarms) and 2011 (seven swarms). These recurrent clusters align with the region’s established pattern of episodic, low-magnitude unrest rather than isolated large events.

Such swarms typically arise from fluid migration or aseismic slip along favorably oriented faults, releasing strain without producing surface rupture. Depths observed in S20120819.1 remain consistent with the brittle-ductile transition zone documented across the Walker Lane. No damage or felt reports were associated with this sequence, reflecting the small magnitudes involved.

Continued monitoring by regional seismic networks supports ongoing assessment of strain accumulation in this tectonically active corridor.

References:
USGS Earthquake Hazards Program – Nevada seismicity summaries
Nevada Bureau of Mines and Geology – Walker Lane tectonic framework
SeismoSight internal swarm classification database