Swarm S20110112.1 was registered in the region 10 km SSE of San Juan Bautista, California. The sequence began at 08:51 on 12 January 2011 and concluded at 21:45 on 16 January 2011, encompassing 108 hours and 54 minutes during which 66 earthquakes occurred.
This swarm unfolded within a tectonically active zone along the San Andreas Fault system. The area lies at the transition between the San Andreas Fault and the Calaveras Fault, where right-lateral strike-slip motion accommodates relative movement between the Pacific and North American plates. Shallow crustal depths, predominantly between 3 km and 9 km, characterized the events, consistent with the brittle deformation typical of this transform boundary.
The initial event registered magnitude 4.5 at 7 km depth. Subsequent activity included multiple events above magnitude 3.0, such as a 3.9 at 7 km depth on 13 January at 04:00:35 and a 3.6 at 7 km depth on 13 January at 03:54:34. Depths remained shallow throughout, with the majority clustered around 5–7 km. Lower-magnitude events dominated the later stages, tapering toward the swarm’s termination.
Historical records indicate six swarms in the region since 1 January 2000. Earlier episodes occurred in 2001 (two swarms), 2002 (one swarm), 2006 (one swarm), 2008 (one swarm), and 2009 (one swarm). These recurrent clusters highlight episodic seismic behavior along the fault segments near San Juan Bautista.
The 2011 swarm contributed to ongoing monitoring of strain accumulation and release in central California. Its compact temporal span and moderate magnitudes align with patterns observed in prior sequences, underscoring the value of continuous seismic surveillance in this high-hazard corridor.
SeismoSight internal swarm classification records.
USGS Earthquake Catalog for regional fault mapping.
California Geological Survey reports on San Andreas Fault history.