Seismic Swarm S20090831.1: Analysis of Activity Near Benton, California

An earthquake swarm designated S20090831.1 was recorded northwest of Benton, California, beginning at 19:21 on 30 August 2009 and concluding at 23:46 on 1 September 2009. The sequence lasted 52 hours and 25 minutes and included 34 events. The epicentral area lies 19 km northwest of Benton in Mono County, within the tectonically active western Great Basin.

The swarm initiated with a magnitude 3.7 earthquake at a depth of 5 km. Subsequent events were predominantly smaller, with magnitudes ranging from 0.0 to 2.4. Depths clustered between 3 km and 8 km, indicating shallow crustal sources. Notable events included a magnitude 2.4 shock on 31 August at 03:17 and a magnitude 2.0 event later that morning. Activity tapered after the initial 24 hours, with only isolated events exceeding magnitude 1.0 on 1 September.

This swarm reflects typical behavior in the region, where clusters of small earthquakes occur without a clear mainshock-aftershock pattern. Such sequences often arise from fluid migration or minor slip along distributed faults within the Walker Lane shear zone. The area experiences ongoing extension and right-lateral shear, producing frequent low-magnitude seismicity.

Since 1 January 2000, five swarms have been documented in the vicinity. Earlier episodes occurred in 2001 (one swarm), 2004 (two swarms), and 2008 (two swarms). These recurrent clusters underscore the persistent seismic character of the local fault network, which accommodates regional strain through episodic bursts rather than isolated large events.

The 2009 swarm's temporal distribution showed a rapid onset followed by a gradual decline, consistent with swarm dynamics driven by aseismic processes. Depths remained stable throughout, suggesting a confined source volume. No events exceeded magnitude 3.7, and the overall energy release remained modest.

Geologically, the site sits amid Basin and Range structures influenced by both tectonic extension and proximity to volcanic centers such as the Long Valley Caldera system. Historical records indicate that similar swarms have occurred in adjacent areas for decades, contributing to the cumulative strain budget without producing damaging ground motions in this instance.

Continued monitoring of such sequences provides insight into the evolving stress field and potential precursors to larger tectonic or volcanic unrest in eastern California.