Seismic Swarm S20000818.1: Analysis of the August 2000 Event Near Benton, California

A seismic swarm designated S20000818.1 was recorded 20 km east of Benton, California, from 22:58 UTC on 17 August 2000 to 07:22 UTC on 19 August 2000. The sequence lasted 32 hours and 24 minutes and comprised 45 earthquakes. This event marks the first swarm documented in the region since systematic recording began in 2000.

The earthquakes exhibited magnitudes between 0.1 and 3.7, with the largest event (magnitude 3.7) occurring at 23:36 on 17 August at a depth of 8 km. Subsequent notable events included multiple magnitude 2.3–2.6 shocks clustered between 23:57 on 17 August and 02:46 on 18 August. Depths ranged predominantly from 5 km to 11 km, consistent with shallow crustal faulting. Activity tapered after 18 August, concluding with a magnitude 2.4 event at 07:22 on 19 August.

The Benton area lies within the Walker Lane belt of eastern California, a zone of distributed right-lateral shear that accommodates approximately 20–25 % of Pacific–North American plate motion. The local geology features active normal and strike-slip faults superimposed on Basin and Range extension, with proximity to the Long Valley Caldera influencing regional stress fields. Earthquake swarms in this setting commonly arise from fluid migration or aseismic slip along fault networks rather than mainshock-aftershock sequences.

Since 1 January 2000, five swarms have been identified in the immediate vicinity, underscoring recurrent episodic seismicity. Such swarms provide valuable data for refining fault models and assessing background hazard levels in this tectonically active corridor.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey Regional Fault Maps
USGS Professional Paper 2151: Seismicity of the Walker Lane Belt