Seismic Swarm S20160319.2: Analysis of Activity Near Floriston, California

A seismic swarm designated S20160319.2 occurred 7 km southeast of Floriston, California, from 19:10 on 18 March 2016 to 15:34 on 19 March 2016. Over 20 hours and 24 minutes, the sequence produced 29 earthquakes, with magnitudes ranging from -0.5 to 2.1 and focal depths between 0 and 10 km. The events clustered in two main phases: an initial burst on the evening of 18 March followed by a secondary peak on the afternoon of 19 March.

The largest event reached magnitude 2.1 at 22:46 on 18 March at 7 km depth. Subsequent activity included multiple events near magnitude 1.0–1.7, with the final recorded shock of magnitude 0.6 occurring at 15:34 on 19 March at 9 km depth. Depths remained predominantly shallow, consistent with upper-crustal faulting in the region.

Floriston lies within the northern Sierra Nevada, near the transition to the Basin and Range province. This area experiences distributed deformation associated with the Walker Lane belt, a zone of dextral shear accommodating part of the Pacific–North American plate motion. Local fault systems, including segments of the Sierra Nevada frontal fault and subsidiary structures, host frequent microseismicity. Swarm activity here typically reflects fluid migration or aseismic slip triggering on small faults rather than mainshock–aftershock sequences driven by a single large rupture.

Since 2000, the Floriston area has recorded 29 swarms. Documented episodes occurred in 2003 (3 swarms), 2004 (2), 2005 (4), 2007 (1), 2008 (7), 2010 (1), 2012 (3), 2013 (2), 2014 (2), and 2015 (4). This pattern indicates recurrent, low-magnitude clustered seismicity without clear long-term escalation.

Such swarms contribute to regional hazard assessment by illuminating active fault networks and stress conditions. Continued monitoring supports refined models of crustal deformation in the northern Sierra Nevada–Walker Lane transition zone.

References

SeismoSight internal swarm catalog (S20160319.2 parameters).
USGS Earthquake Catalog (regional background seismicity).
California Geological Survey, Quaternary Fault and Fold Database (tectonic setting).