Seismic Swarm S20080411.1: The 2008 Mogul, Nevada Sequence

The seismic swarm designated S20080411.1 began at 00:18 on 11 April 2008 and concluded at 20:37 on 5 June 2008, centered 4 km east-northeast of Mogul, Nevada. Over 1340 hours and 19 minutes, the sequence produced 13,408 earthquakes. This prolonged, high-rate episode of microseismicity exemplifies swarm behavior typical of the region, where fluid migration and fault interactions often sustain activity without a dominant mainshock.

Mogul lies within the northern Walker Lane, a zone of distributed dextral shear and extension east of the Sierra Nevada. The area forms part of the Basin and Range Province, where northwest-trending normal and strike-slip faults accommodate ongoing tectonic deformation at rates of several millimeters per year. Shallow crustal seismicity is common, with most events occurring above 5 km depth due to the brittle nature of the upper crust in this extensional regime.

Analysis of the first 100 events reveals predominantly microearthquakes with magnitudes between −0.5 and 2.2. Depths remained very shallow, clustered between 0 and 3 km, with only a few events reaching 6 km. The initial hours showed rapid onset of activity, including multiple events within minutes, followed by a steady rate of small-magnitude releases. Notable early events included a magnitude 1.4 at 12:17 on 11 April and a magnitude 2.2 at 12:24 on 12 April, both at depths of 0–1 km. Such patterns indicate distributed slip on closely spaced fractures rather than rupture along a single major fault plane.

Historical records document nine swarms in the Mogul area since 1 January 2000. Prior episodes occurred in 2003 (1 swarm), 2004 (2 swarms), 2005 (3 swarms), 2007 (1 swarm), and 2008 (2 swarms). These recurrent sequences highlight the persistent seismic productivity of local fault networks, driven by regional extension and possible hydrothermal influences.

The 2008 swarm contributed to refined understanding of short-term seismic hazards in the Reno–Mogul corridor. Its shallow focus and high event count underscore the potential for felt shaking from even modest-magnitude events in populated areas. Continued monitoring supports improved models of strain accumulation along Walker Lane structures.

References

SeismoSight internal swarm catalog (S20080411.1 parameters and event list).
USGS Earthquake Catalog (regional seismicity and Walker Lane tectonics).
Nevada Seismological Laboratory reports on Mogul-area sequences.