Analysis of Seismic Swarm S20130827.1 near Spanish Springs, Nevada

Seismic swarm S20130827.1 occurred in a tectonically active region 2 km northeast of Spanish Springs, Nevada. The sequence began at 09:37 on 26 August 2013 and concluded at 08:16 on 26 September 2013, spanning 742 hours and 38 minutes. During this period, 944 earthquakes were recorded.

The swarm took place within the Basin and Range Province, where crustal extension produces normal faulting and distributed seismicity. Spanish Springs lies near the northern termination of the Walker Lane shear zone, a transitional belt accommodating right-lateral shear between the Pacific and North American plates. This setting favors episodic earthquake swarms rather than large mainshock-aftershock sequences.

Examination of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from -0.4 to 4.2, with the majority below 1.0. The largest event, magnitude 4.2, occurred at 00:51:43 on 27 August at a depth of 13 km. Focal depths clustered between 9 km and 14 km, consistent with brittle failure in the mid-crust of this extensional regime. Early activity included several negative-magnitude events, indicating a high detection threshold for microseismicity.

Subsequent events maintained similar depth ranges while magnitudes gradually declined after the initial peak. This pattern aligns with fluid-driven swarm behavior commonly observed in the western Great Basin, where pore-pressure changes can trigger prolonged, diffuse seismicity without a dominant mainshock.

Historical records since 1 January 2000 document 13 swarms in the immediate area. Earlier episodes occurred in 2008 (six swarms), 2012 (one swarm), and 2013 (six swarms). The recurrence underscores the persistent seismic productivity of the Spanish Springs vicinity.

The geological context explains the swarm characteristics. Regional extension rates of several millimeters per year, combined with Quaternary fault scarps, indicate ongoing strain accumulation. Swarm durations of weeks to months are typical where hydrothermal or magmatic fluids interact with favorably oriented faults.

No surface rupture or significant damage was associated with S20130827.1, consistent with its modest energy release. Continued monitoring remains essential given the proximity to the Reno-Sparks metropolitan area.

References

• U.S. Geological Survey Earthquake Catalog
• Nevada Seismological Laboratory swarm database
• USGS Professional Paper on Walker Lane tectonics