Seismic Swarm S20040603.1: Analysis of Activity Near Floriston, California

The seismic swarm designated S20040603.1 occurred 7 km south-southeast of Floriston, California, beginning at 14:55 on 2 June 2004 and concluding at 15:04 on 10 June 2004. Over 192 hours and 9 minutes, 197 earthquakes were recorded. This event represents one of three swarms documented in the region since 1 January 2000, following an initial swarm in 2003. Analysis of the first 100 events reveals predominantly low-magnitude activity clustered at shallow depths. Magnitudes ranged from -0.3 to 4.2, with the majority below 1.0. Depths varied between 2 km and 11 km, centering around 5–7 km. The sequence initiated with a 0.3-magnitude event at 4 km depth, followed by a gradual increase in frequency. On 3 June, activity intensified, including events of 2.4, 3.0, and a peak 4.2 magnitude at 2 km depth. Subsequent events maintained a pattern of frequent micro-earthquakes interspersed with occasional moderate shocks up to 1.8 magnitude. Depths remained consistent in the upper crust, suggesting a localized source zone. The Floriston area lies within the northern Sierra Nevada, near the California-Nevada border. This region experiences tectonic strain from the broader Pacific–North American plate boundary, modulated by the Walker Lane shear zone and Basin and Range extension. Fault systems in the vicinity accommodate right-lateral and normal slip, producing shallow crustal seismicity. Historical records indicate recurrent low-to-moderate earthquake swarms driven by fluid migration or stress transfer along these structures. The 2004 swarm aligns with established patterns of episodic unrest in this seismically active corridor. Depths predominantly under 10 km are typical for the local geology, where brittle failure occurs in the upper crust above a ductile transition zone. No surface rupture was associated with the sequence, consistent with the small event sizes. References
SeismoSight internal swarm classification records.
USGS Earthquake Catalog (regional tectonic framework).
California Geological Survey (Sierra Nevada fault history).