A seismic swarm designated S20111220.1 occurred 23 km northeast of Soledad in Monterey County, California. The sequence began at 07:08 on 19 December 2011 and concluded at 10:09 on 22 December 2011, spanning 75 hours and 1 minute. During this interval, 59 earthquakes were recorded, with magnitudes ranging from 0.4 to 3.0 and focal depths predominantly between 1 and 6 km.
The events clustered temporally, with the highest activity on 19 and 20 December. Notable events included a magnitude 2.8 quake at 20:20 on 19 December at 4 km depth, a magnitude 3.0 event at 05:06 on 20 December at 4 km depth, and a magnitude 2.7 quake at 17:44 on 20 December at 4 km depth. Most events remained shallow, consistent with swarm behavior driven by fluid migration or localized stress adjustments rather than a single large rupture.
This swarm fits established patterns in the region. Since 2000, 13 swarms have occurred in the area, distributed across years as follows: one each in 2000, 2003, 2004, 2005, 2008, 2009, and 2011; two each in 2001, 2006, and 2007. Such recurrent swarms reflect ongoing tectonic strain accumulation along the Pacific-North American plate boundary.
The Soledad area lies within the central California Coast Ranges, where right-lateral strike-slip faulting dominates. The nearby San Andreas Fault system accommodates approximately 35 mm per year of relative plate motion. Local structures include the Sargent and Calaveras fault zones, which exhibit microseismicity and occasional swarm activity due to their proximity to the main San Andreas trace. Historical records document major ruptures in 1857 (Fort Tejon earthquake, magnitude approximately 7.9) and 1906 (San Francisco earthquake, magnitude 7.8), both of which influenced regional stress fields still relevant today.
Contemporary monitoring by the USGS confirms that the central Coast Ranges experience persistent low-magnitude seismicity. Depths in this swarm align with the brittle-ductile transition zone in the upper crust, where temperatures permit brittle failure above roughly 10 km. No surface rupture or damage was associated with the 2011 sequence, typical for events below magnitude 4.
Insights from the temporal and spatial distribution indicate swarm-like characteristics: no dominant mainshock-aftershock decay, rapid onset and cessation, and uniform magnitude-frequency distribution. These features suggest pore-pressure diffusion or aseismic slip triggering as plausible mechanisms, common in transform-margin settings.