Seismic swarm S20120406.1 occurred approximately 24 km northeast of Greenfield in Monterey County, California. The sequence began at 21:47 UTC on 5 April 2012 and concluded at 06:01 UTC on 11 April 2012, spanning 128 hours and 13 minutes. During this interval, 55 earthquakes were recorded, with magnitudes ranging from 0.5 to 3.6 and focal depths between 2 km and 10 km. The largest event, magnitude 3.6, occurred at 02:55 UTC on 6 April at a depth of 5 km. Subsequent notable events included several magnitude 2.0–2.5 shocks clustered between 6 and 8 April, primarily at depths of 4–5 km. The majority of activity remained below magnitude 2.0, consistent with typical swarm characteristics where energy release occurs through numerous small events rather than a dominant mainshock followed by aftershocks. The swarm region lies within the central California Coast Ranges, part of the broader San Andreas Fault system. This tectonically active zone accommodates right-lateral strike-slip motion between the Pacific and North American plates at a rate of approximately 35–38 mm per year. Local faulting includes strands of the San Andreas Fault itself and subsidiary structures such as the Calaveras and Sargent faults to the north. Crustal deformation in the area is influenced by both regional plate motion and local restraining bends that can promote distributed seismicity. Earthquake swarms in central California often reflect transient stress changes, possibly driven by aseismic slip or fluid migration along fault zones. Depths recorded during this swarm (mostly 3–7 km) align with the brittle-ductile transition zone in this portion of the San Andreas system, where small-magnitude events are common. No surface rupture or significant damage was associated with the sequence. Historical records indicate recurrent swarm activity in the region. Since 1 January 2000, 14 swarms have been documented near Greenfield, occurring in the years 2000 (1), 2001 (2), 2003 (1), 2004 (1), 2005 (1), 2006 (2), 2007 (2), 2008 (1), 2009 (1), and 2011 (2). This pattern suggests episodic strain release along the same fault network.
USGS Earthquake Hazards Program – San Andreas Fault System overview
California Geological Survey – Regional fault and seismicity maps
USGS Earthquake Catalog (general tectonic framework)