Earthquake Swarm S20140623.1 Near Floriston, California

The earthquake swarm designated S20140623.1 occurred approximately 4 km south-southeast of Floriston in Nevada County, California. It began at 20:33 UTC on 22 June 2014 and concluded at 16:17 UTC on 24 June 2014, spanning 43 hours and 43 minutes. During this interval, 28 earthquakes were recorded, with magnitudes ranging from -0.8 to 2.2 and focal depths between -2 km and 13 km.

Events initiated with a magnitude -0.1 shock at 6 km depth, followed by several small tremors clustered near the surface and at shallow depths. The largest event, magnitude 2.2, occurred at 05:48 on 23 June at 6 km depth, accompanied shortly afterward by a magnitude 1.2 event at the same depth. Subsequent activity included multiple events near or above 1.0 in magnitude, such as the 1.5 events recorded at 06:46 and 18:13 on 23 June. Depths varied notably, with some events registered at negative values indicating very shallow or near-surface sources, while others reached up to 13 km. Activity tapered off by the afternoon of 24 June with a final magnitude 0.8 event at 5 km depth.

This swarm reflects the typical pattern of clustered, low-magnitude seismicity common in the region. The Floriston area lies within the northern Sierra Nevada, near the transition to the Basin and Range province and the Walker Lane shear zone. These tectonic settings feature active normal and strike-slip faulting driven by regional extension and right-lateral shear. The local geology comprises Mesozoic granitic rocks of the Sierra Nevada batholith overlain by Cenozoic volcanic and sedimentary deposits, with numerous mapped faults capable of producing small earthquakes.

Historical records indicate 23 swarms have occurred in the area since 1 January 2000. Earlier swarms were documented in 2003 (3 events), 2004 (2), 2005 (4), 2007 (1), 2008 (7), 2010 (1), 2012 (3), 2013 (1), and 2014 (1). Such recurrent swarms underscore the ongoing tectonic adjustment along regional fault systems without evidence of a single dominant mainshock-aftershock sequence.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey Fault Activity Map
Nevada Seismological Laboratory regional reports