An earthquake swarm designated S20090120.1 occurred 10 km south of Tres Pinos in San Benito County, California. The sequence began at 18:27 on 19 January 2009 and concluded at 23:44 on 22 January 2009, spanning 77 hours and 16 minutes. During this interval, 68 earthquakes were recorded, with magnitudes ranging from 0.1 to 3.9 and focal depths primarily between 3 km and 10 km.
Tres Pinos lies within the Coast Ranges physiographic province, where the interaction between the Pacific and North American tectonic plates drives ongoing deformation. The region sits near the Calaveras Fault and associated strands of the San Andreas Fault system, which accommodate right-lateral strike-slip motion. This tectonic setting produces both large-magnitude events and frequent smaller sequences, including swarms. Historical records indicate that swarm activity has been documented in the broader area since at least the early 2000s, with ten such episodes recorded between 2000 and 2008.
The 2009 swarm initiated with a magnitude 1.0 event at 3 km depth. Activity intensified on 20 January, featuring several events above magnitude 3.0, including a 3.9 quake at 6 km depth and multiple 3.8 events. Magnitudes generally decreased after the initial peak, though isolated events above magnitude 2.0 continued through 22 January. Depths remained shallow throughout, consistent with activity in the upper crust of this fault zone.
Such swarms differ from typical aftershock sequences by lacking a single dominant mainshock. Instead, they reflect distributed stress release along fault segments, often linked to fluid migration or aseismic creep common along the San Andreas system near Hollister and Tres Pinos. The 2009 sequence aligns with this pattern, showing clustered energy release over several days without escalation to a larger rupture.
Regional geology features Franciscan Complex bedrock overlain by Cenozoic sedimentary units, folded and faulted by plate-boundary tectonics. Paleoseismic studies reveal recurrent slip on nearby faults over Holocene time, underscoring the area's persistent seismic hazard. Updated monitoring by regional networks continues to track microseismicity, aiding understanding of strain accumulation.
This swarm contributes to the documented history of episodic activity south of Tres Pinos. With prior swarms occurring in 2000, 2001, 2003, 2005, 2006, 2007, and 2008, the 2009 event reinforces the characteristic behavior of this portion of the fault system.
United States Geological Survey Earthquake Hazards Program.
California Geological Survey Fault Activity Map.
Working Group on California Earthquake Probabilities reports.