Seismic Swarm Analysis: Sun Valley, Nevada – March 2013

A seismic swarm designated S20130304.1 occurred 2 km east of Sun Valley, Nevada, between 08:25 on 3 March 2013 and 12:27 on 4 March 2013. Over 28 hours and 1 minute, the sequence produced 32 earthquakes, all of low magnitude and shallow depth. The events clustered tightly in both space and time, consistent with swarm behavior rather than a classic mainshock-aftershock sequence.

Event parameters show magnitudes ranging from –0.8 to 1.7 and focal depths between 9 and 14 km. The largest event (M 1.7) occurred at 11:10:31 on 3 March at 11 km depth. Negative magnitudes indicate microseismicity detectable only by sensitive local networks. Depths remained stable throughout, suggesting a single, limited source volume within the brittle upper crust.

Regional Geological Setting

Sun Valley lies within the northern Basin and Range Province, where east-west extension is accommodated by normal faulting along north-striking range-front faults. The area forms part of the Walker Lane belt, a zone of distributed shear that transfers Pacific–North America plate motion. Crustal thickness averages 30–35 km, and heat flow is elevated, promoting brittle failure at shallow depths. Quaternary fault scarps and ongoing geodetic strain rates of several millimeters per year confirm active tectonics. Small-magnitude swarms are common and typically linked to fluid migration or aseismic slip transients on favorably oriented faults.

Historical Context

Since 1 January 2000, eight swarms have been recorded in the immediate vicinity. Earlier episodes occurred in 2008 (six swarms), 2010 (one swarm), and 2012 (one swarm). These recurrent clusters indicate that the crust beneath Sun Valley is predisposed to swarm-type activity, likely controlled by the same fault network that hosted the 2013 sequence.

Event Insights

The 2013 swarm exhibited a rapid onset followed by a gradual decline in rate, with most events occurring within the first 12 hours. Magnitude distribution remained narrow, and no event exceeded M 2.0, limiting felt reports. Depth consistency across the catalog points to a compact source zone approximately 5 km in vertical extent. Such patterns align with known swarm mechanics in the Reno–Carson City corridor, where episodic fluid pressure or slow slip can trigger prolonged sequences without a dominant mainshock.

Continued seismic monitoring in this region contributes to refined hazard models for the rapidly urbanizing Truckee Meadows area. The 2013 swarm underscores the value of dense local networks in characterizing low-level activity that may otherwise go undetected.

References

USGS Earthquake Catalog (ComCat)
Nevada Seismological Laboratory, University of Nevada, Reno
USGS Quaternary Fault and Fold Database of the United States
GPS velocity fields from the Nevada Geodetic Laboratory