Seismic Swarm S20091205.1: Analysis of Earthquake Activity Near Beatty, Nevada

A seismic swarm designated S20091205.1 occurred approximately 50 km east-southeast of Beatty, Nevada, from 08:01 on 4 December 2009 to 15:48 on 6 December 2009. Over 55 hours and 46 minutes, the event sequence registered 34 earthquakes. All events were of low magnitude, ranging from -1.4 to 0.5, with focal depths predominantly between 2 km and 11 km. The swarm exhibited a typical pattern of clustered microseismicity without a dominant mainshock, consistent with fluid-driven or stress-triggered activity in an extensional tectonic setting.

The sequence began with events of magnitude 0.1 and 0.0 on 4 December, followed by a series of negative-magnitude detections clustered around midday. Activity continued at a low level through the afternoon and evening, with magnitudes remaining below 0.5. On 5 December, the largest event of the swarm (magnitude 0.5) occurred at 00:42, accompanied by additional small events at depths as shallow as 2 km. The final events on 6 December included magnitudes of 0.0, 0.4, and 0.3. Depths showed minor variation, with most events between 7 km and 10 km, suggesting rupture within the upper crust.

This swarm fits into a broader pattern of seismic activity in the region. Since 1 January 2000, 29 swarms have been documented in the same area. Yearly counts include five swarms in 2000, seven in 2002, two in 2003, two in 2005, one in 2006, two in 2007, seven in 2008, and three in 2009. Such recurrent swarms reflect ongoing tectonic processes rather than isolated aftershock sequences.

The location lies within the Basin and Range Province, where east-west extension produces normal faulting and distributed seismicity. Beatty sits near the southern margin of this province, adjacent to the Walker Lane shear zone that accommodates a portion of Pacific-North America plate motion. Crustal extension in this area occurs at rates of several millimeters per year, with fault systems capable of generating both small swarms and larger events. Historical seismicity includes moderate earthquakes, underscoring the region's elevated hazard level despite the low magnitudes observed in this particular swarm.

Microearthquake swarms like S20091205.1 often result from transient increases in pore-fluid pressure or aseismic slip that load nearby faults. The shallow depths and tight spatiotemporal clustering observed here align with these mechanisms. No damage or felt reports were associated with the sequence, as expected for events below magnitude 1.0.

Ongoing monitoring of such swarms contributes to refined seismic hazard assessments for central Nevada. The frequency of swarm activity since 2000 indicates persistent strain accumulation that warrants continued observation by regional networks.

References

USGS Earthquake Hazards Program – Nevada Seismicity
Nevada Bureau of Mines and Geology – Regional Tectonic Framework
SeismoSight internal swarm catalog (S20091205.1 parameters)