Seismic Swarm Analysis: Hawthorne, Nevada Region, January 2011

A seismic swarm designated S20110123.1 occurred approximately 20 km east of Hawthorne, Nevada, between 16:48 UTC on 22 January 2011 and 07:05 UTC on 23 January 2011. Over 14 hours and 16 minutes, 27 earthquakes were recorded, with magnitudes ranging from 0.9 to 4.2 and focal depths primarily between 5 km and 14 km. The sequence featured two events exceeding magnitude 3.9, highlighting clustered microseismicity typical of the region's active fault systems.

The Hawthorne area lies within the western Basin and Range province, specifically influenced by the Walker Lane tectonic belt. This zone accommodates dextral shear between the Pacific and North American plates, resulting in northwest-trending strike-slip faults and north-south normal faults that produce frequent earthquake swarms. Extensional tectonics have shaped the landscape through repeated faulting since the Miocene, creating the characteristic basin-and-range topography. Depths recorded during the swarm align with brittle failure in the upper crust, consistent with regional seismogenic depths of 5–15 km.

Historical seismicity in Mineral County includes multiple swarms and moderate events linked to the same fault network. The 1932 Cedar Mountain earthquake (magnitude 7.2) occurred roughly 80 km northeast, demonstrating the capacity for larger ruptures along similar structures. More recent activity, such as swarms in the 1990s and 2000s near the California-Nevada border, reflects ongoing strain accumulation at rates of 8–12 mm per year across the Walker Lane. Updated assessments from regional monitoring networks confirm persistent low-to-moderate seismicity east of Hawthorne, underscoring the area's elevated hazard relative to stable continental interiors.

The 2011 swarm initiated with a magnitude 1.6 event at 9 km depth, rapidly escalating to a magnitude 4.2 shock at 8 km depth within 41 minutes. Subsequent events included a magnitude 3.9 at 7 km depth and another magnitude 3.9 at 7 km depth later that evening. Magnitudes generally decreased after the initial peaks, with most events clustering between 6 km and 11 km depth. One outlier at 42 km depth suggests possible deeper crustal involvement or location uncertainty. The temporal pattern—intense activity in the first four hours followed by sporadic smaller events—typifies fluid-driven or stress-triggered swarms rather than mainshock-aftershock sequences.

Such swarms contribute to hazard evaluation by revealing active fault segments and stress orientations. In the Hawthorne vicinity, they often occur along unmapped or secondary faults subsidiary to the larger Walker Lane structures. No surface rupture was associated with this event, consistent with its modest energy release. Monitoring data indicate that similar swarms recur every few years, providing valuable constraints on recurrence intervals for larger earthquakes.

Regional geology features Quaternary alluvium overlying Paleozoic and Mesozoic bedrock, with active fault scarps visible in surrounding ranges. Geothermal activity and young volcanic features nearby further indicate elevated heat flow that may facilitate swarm occurrence through pore-pressure changes.

References

• USGS Earthquake Hazards Program regional reports on Nevada seismicity
• Nevada Bureau of Mines and Geology tectonic summaries
• Walker Lane strain-rate studies from geodetic networks (updated through 2023)