Earthquake swarm S20071128.1 occurred in a tectonically active zone 24 km northeast of Soledad, California, within the Salinas Valley region of the California Coast Ranges. This area forms part of the broader Pacific-North American plate boundary, where right-lateral strike-slip faulting predominates along the San Andreas Fault system and associated secondary structures. The local geology features sedimentary basins overlying Franciscan Complex basement rocks, with ongoing deformation driven by northwestward plate motion. Depths of recorded events typically ranged from 1 to 11 km, consistent with shallow crustal seismicity in this segment of central California.
The swarm initiated at 02:30 on 28 November 2007 and concluded at 23:29 on 7 December 2007, spanning 236 hours and 59 minutes. A total of 183 earthquakes were registered during this interval. Analysis of the first 100 events reveals a pattern of clustered activity beginning with a magnitude 3.4 event at 8 km depth. Subsequent events included multiple magnitude 2+ shocks, such as 2.9 at 4 km, 2.2 at 4 km, and 3.2 at 4 km on 29 November, interspersed with numerous microearthquakes below magnitude 1.0. Depths remained predominantly shallow, with many events between 3 and 7 km, and activity showed temporal clustering with bursts of events separated by quieter intervals. Magnitudes generally declined after the initial larger shocks, reflecting typical swarm behavior without a dominant mainshock-aftershock sequence.
Historical records indicate seven prior swarms in the region since 1 January 2000. These occurred in 2000 (1 swarm), 2001 (1), 2003 (1), 2004 (1), 2005 (1), and 2006 (2). Such recurrent swarms underscore the area's persistent low-to-moderate seismic productivity, often linked to fluid migration or stress transfer along local fault networks.
This swarm highlights the value of continuous monitoring for understanding episodic seismic release in central California. Data from the sequence contribute to refined models of regional fault interactions and hazard assessment.
References SeismoSight internal classification records for swarm S20071128.1 USGS Earthquake Hazards Program reports on central California seismicity California Geological Survey bulletins on Coast Ranges tectonics