Earthquake Swarm S20150408.1: Analysis of Seismicity Near Floriston, California

The earthquake swarm designated S20150408.1 occurred 3 km south of Floriston, California, in the tectonically active northern Sierra Nevada region. This area lies near the intersection of the Sierra Nevada frontal fault system and the Walker Lane belt, a zone of distributed dextral shear accommodating a portion of the Pacific-North American plate motion. The local geology features Mesozoic granitic intrusions of the Sierra Nevada batholith overlain by Tertiary volcanic and sedimentary units, with active normal and strike-slip faults contributing to regional extension and seismicity.

The swarm initiated at 01:50 on 8 April 2015 and concluded at 11:37 on 10 April 2015, spanning 57 hours and 47 minutes. During this interval, 57 earthquakes were recorded. Event magnitudes ranged from -0.5 to 3.4, with the largest shock (magnitude 3.4) occurring at 05:06 on 8 April at a depth of 0 km. Depths varied from -1 km to 10 km, indicating a shallow crustal source consistent with the area's fault architecture. Notable events included a magnitude 2.8 quake at 23:56 on 8 April and a magnitude 2.1 event at 06:47 on the same day, both at shallow depths.

Temporal distribution showed peak activity in the initial hours, with 35 events on 8 April tapering to 18 on 9 April and 4 on 10 April. Many events clustered within the first 12 hours, reflecting rapid stress release along a localized fault segment. Negative magnitudes and depths near 0 km are typical of microseismicity in this volcanic-influenced terrain.

Since 1 January 2000, 28 swarms have been documented in the Floriston vicinity. Annual counts include three in 2003, two in 2004, four in 2005, one in 2007, seven in 2008, one in 2010, three in 2012, four in 2013, two in 2014, and one in 2015. This pattern underscores recurring swarm behavior driven by fluid migration or aseismic slip on pre-existing faults rather than mainshock-aftershock sequences.

Such swarms provide insight into the region's low-to-moderate seismic hazard. While individual events rarely exceed magnitude 4, their clustered nature can produce perceptible shaking and inform probabilistic models for the northern Sierra Nevada.

References

SeismoSight internal swarm catalog (S20150408.1 parameters and historical statistics)
USGS Earthquake Catalog (regional seismicity and fault data)
California Geological Survey (Sierra Nevada tectonics and geology)