Seismic Swarm S20081121.2: Analysis of Activity near Floriston, California

A seismic swarm designated S20081121.2 was recorded 4 km SSE of Floriston, California, beginning at 01:40 on 21 November 2008 and concluding at 18:45 on 22 November 2008. Over 41 hours and 5 minutes, the sequence produced 26 earthquakes with magnitudes ranging from 0.5 to 2.5 and focal depths between 0 and 10 km. The largest event reached magnitude 2.5 at a depth of 2 km on 21 November at 20:34:15 UTC. Most events clustered between 2 and 9 km depth, consistent with shallow crustal processes in the region.

The swarm exhibited a typical pattern of clustered, low-magnitude seismicity without a dominant mainshock-aftershock sequence. Early activity on 21 November included events of magnitude 1.6, 1.2, and 1.1 within the first few hours, followed by a peak period in the evening with multiple events above magnitude 1.0. Activity tapered on 22 November with two final magnitude-1.5 events. Depths remained predominantly in the upper crust, suggesting involvement of near-surface fault structures.

Floriston lies within the northern Sierra Nevada, where tectonic deformation is driven by the interaction between the Pacific and North American plates and the transtensional regime of the Walker Lane belt. This setting produces distributed normal and strike-slip faulting, contributing to recurrent low-level seismicity. Historical records since 2000 indicate 16 swarms in the immediate area, with notable occurrences in 2003 (3 swarms), 2004 (2), 2005 (4), 2007 (1), and 2008 (6). These episodes reflect episodic strain release along local fault networks rather than large-scale plate-boundary rupture.

Such swarms are common in the Basin and Range transition zone east of the Sierra Nevada crest and rarely exceed magnitude 3.0. The 2008 sequence aligns with this pattern, posing minimal hazard but providing data on fault behavior and fluid migration that can influence swarm evolution. Continued monitoring supports improved understanding of seismic hazard in this tectonically active corridor.

References

USGS Earthquake Catalog (events 2000–2008)
Nevada Seismological Laboratory regional reports
California Geological Survey fault database