Seismic Swarm S20000801.1 Near Benton, California: Geological Context and Event Analysis

The region 22 km east of Benton in Mono County, California, forms part of the Basin and Range province, where crustal extension drives normal faulting along north-south trending structures. This tectonic setting produces frequent small-magnitude earthquakes, often occurring in clusters known as swarms. The area lies near the transition between the Sierra Nevada block and the Walker Lane belt, with historical seismic activity influenced by both regional extension and proximity to volcanic features such as the Long Valley Caldera approximately 50 km to the northwest.

Seismic swarms in this part of eastern California typically involve numerous events over hours to days without a single dominant mainshock. Swarm S20000801.1, the first recorded in the SeismoSight database since January 2000, began at 20:50 on 31 July 2000 and concluded at 22:37 on 1 August 2000. Over 25 hours and 47 minutes, the sequence included 31 earthquakes with magnitudes ranging from 0.3 to 2.0 and focal depths between 5 km and 11 km.

The sequence opened with a magnitude 1.3 event at 6 km depth. Subsequent activity showed a gradual increase in event rate during the afternoon of 1 August, peaking with multiple events between 13:41 and 13:56, including magnitudes up to 1.5. The largest event, magnitude 2.0, occurred at 15:53 at 5 km depth. Later events remained shallow, with the final recorded earthquake of magnitude 0.9 at 5 km depth marking the swarm's end. Depths clustered predominantly between 5 km and 10 km, consistent with brittle failure in the upper crust of this extensional regime.

Such swarms provide insight into fluid migration or aseismic slip along fault zones, common in the tectonically active Basin and Range. Since 2000, three additional swarms have been documented in the same classification system, underscoring the recurrent nature of clustered seismicity east of Benton.

References

USGS Earthquake Hazards Program regional tectonic summaries for Mono County, California.
California Geological Survey fault activity maps and Basin and Range province descriptions.
SeismoSight internal swarm classification records for event S20000801.1.