Seismic Swarm S20091114.1 in Nevada: Analysis of the November 2009 Event

Seismic swarms represent clusters of earthquakes occurring in a localized area over a short period without a dominant mainshock. Swarm S20091114.1 was recorded in Nevada from 09:08 on 13 November 2009 to 19:42 on 17 November 2009. Over 106 hours and 34 minutes, 95 earthquakes were detected, with magnitudes ranging from -1.5 to 2.5 and focal depths primarily between 0 and 8 km.

Activity began with low-magnitude events on 13 November, including a magnitude 1.0 quake at 5 km depth. Intensity increased on 14 November, featuring several events above magnitude 1.0, such as a magnitude 2.5 at 5 km depth at 08:19 and a magnitude 2.0 at 5 km depth at 08:58. Additional notable shocks included a magnitude 2.3 at 5 km depth later that day. Subsequent days showed declining frequency and amplitude, with the final event recorded at magnitude 0.1 on 17 November.

Nevada lies within the Basin and Range Province, a region of active extensional tectonics driven by the interaction between the Pacific and North American plates. This setting produces widespread normal faulting and is responsible for elevated seismicity across the state. The Walker Lane shear zone, a transtensional belt east of the Sierra Nevada, further contributes to strain accumulation and release through frequent small-magnitude events and occasional swarms.

Historical records indicate that Nevada has experienced multiple seismic swarms since 2000, with documented episodes in 2000, 2002, 2005, 2008, and 2009. These episodes typically involve shallow events consistent with the regional fault architecture.

The 2009 swarm exhibited characteristics common to Basin and Range activity, with most events at depths of 3–5 km and a rapid onset followed by gradual decay. Such patterns aid in understanding fluid migration and stress transfer along local fault networks.

References

• Nevada Seismological Laboratory, University of Nevada, Reno – Regional seismic monitoring reports.
• U.S. Geological Survey Earthquake Hazards Program – Basin and Range Province tectonic summaries (updated through 2023).
• Western United States seismic catalogs for swarm frequency statistics.