An earthquake swarm designated S20060308.1 was recorded 21 km northeast of Soledad, California, in Monterey County. The sequence began at 00:41 on 8 March 2006 and concluded at 11:40 on 10 March 2006, lasting 58 hours and 58 minutes. During this period, 39 earthquakes were detected, with magnitudes ranging from 0.4 to 3.0 and focal depths primarily between 2 km and 9 km.
The swarm exhibited typical characteristics of clustered seismic activity, with the largest event (magnitude 3.0) occurring early on 8 March at a depth of 5 km. Subsequent events showed a gradual decline in frequency and intensity, including a magnitude 2.6 event on 10 March at 3 km depth. Depths remained shallow throughout, consistent with activity in the upper crust. No events exceeded magnitude 3.0, and the majority registered below magnitude 2.0.
The location lies within the central Coast Ranges of California, a region shaped by the interaction of the Pacific and North American plates along the San Andreas Fault system. Soledad sits in the Salinas Valley, where Quaternary alluvial deposits overlie older sedimentary and metamorphic rocks. The area experiences distributed deformation from the San Andreas Fault, located approximately 30 km to the east, as well as subsidiary faults such as the Calaveras and Zayante-Vergeles systems. Historical seismicity in this zone reflects both strike-slip motion and occasional normal faulting in releasing bends.
Seismic swarms in this part of California often relate to fluid migration or aseismic slip along fault networks rather than mainshock-aftershock sequences. The 2006 swarm fits this pattern, with events distributed over a compact area and no dominant rupture. Regional monitoring by the USGS and regional networks has documented similar low-magnitude clusters in the Salinas Valley since the late twentieth century.
Since 1 January 2000, six swarms have been identified in the immediate vicinity. Prior episodes occurred in 2000 (1 swarm), 2001 (2 swarms), 2003 (1 swarm), 2004 (1 swarm), and 2005 (1 swarm). These recurrent clusters indicate persistent low-level strain accumulation along local fault segments.
The provided event catalog shows a concentration of activity on 8 March, followed by reduced rates on 9 and 10 March. Magnitudes remained modest, and depths clustered around 4–6 km, suggesting a shallow source volume. Such patterns assist in characterizing background seismicity and refining hazard assessments for the central California coast.
SeismoSight internal swarm classification S20060308.1
USGS Earthquake Catalog (regional data for Monterey County, California)
California Geological Survey, Quaternary Fault and Fold Database