Seismic swarm S20060909.1 occurred 5 km southwest of Ridgemark in San Benito County, California. The sequence began at 18:54 on 8 September 2006 and concluded at 18:54 on 10 September 2006, spanning 47 hours and 59 minutes. During this interval, 63 earthquakes were recorded.
The swarm exhibited typical characteristics of low-magnitude, clustered seismicity. Event magnitudes ranged from 0.6 to 2.6, with the majority below 2.0. Depths concentrated between 8 and 11 km, though several shallower events occurred near 2–6 km. The sequence initiated with a magnitude 0.8 event at 2 km depth, followed by progressive activity that peaked around 00:42 on 9 September with a magnitude 2.6 shock at 9 km. Later events on 9 and 10 September showed a gradual decline in both frequency and magnitude, consistent with swarm behavior rather than a mainshock-aftershock pattern.
Such swarms reflect fluid migration or aseismic slip along fault segments within the broader San Andreas system. The Ridgemark area lies in the California Coast Ranges, where right-lateral strike-slip faults accommodate Pacific-North American plate motion. Historical records indicate four swarms in the region since 1 January 2000, occurring in 2001 (two events), 2002 (one event), and 2006 (this sequence). These episodes underscore persistent low-level seismic unrest without producing damaging ground motions.
The local geology features Quaternary sediments overlying Franciscan Complex basement rocks, cut by strands of the Calaveras and San Andreas faults. Ongoing creep and microseismicity along these structures release strain gradually, reducing the likelihood of large single events but sustaining swarm activity.