A seismic swarm designated S20250222.3 occurred 11 km south-southwest of Tres Pinos in San Benito County, California. The sequence began at 05:18 on 22 February 2025 and concluded at 09:21 on 23 February 2025, spanning 28 hours and 3 minutes. During this period, 26 earthquakes were recorded.
The events were predominantly shallow, with focal depths ranging from 1 to 7 km and most clustered between 3 and 4 km. Magnitudes varied from 0.8 to 3.8, with the largest event reaching 3.8 at 10:31 on 22 February. Activity showed temporal clustering, including multiple events within minutes during peak periods such as the afternoon of 22 February. Smaller events below magnitude 2.0 comprised a significant portion of the sequence, consistent with swarm characteristics where energy release occurs through numerous moderate shocks rather than a single mainshock-aftershock pattern.
This swarm aligns with the tectonic setting of the central Coast Ranges, where the region experiences deformation driven by the San Andreas Fault system. Tres Pinos lies near the Calaveras Fault, a major right-lateral strike-slip structure that accommodates a portion of the Pacific-North American plate boundary motion. The local geology features Franciscan Complex rocks and sedimentary units of the Great Valley Sequence, which are heavily faulted and folded due to ongoing transpression.
Seismic swarms have been recurrent in this area. Historical records since 2000 document 22 such swarms, distributed across years as follows: one each in 2000, 2003, 2005, 2008, 2009, 2012, 2014, 2017, 2018, 2019, 2021, and 2022; two each in 2001 and 2007; three each in 2006 and 2011. These episodes reflect the fault system's segmented behavior and fluid migration influences that can trigger clustered seismicity without large-magnitude ruptures.
The February 2025 swarm contributes to ongoing monitoring of strain accumulation along the Calaveras Fault, which last produced a significant event in the 1980s. Continuous seismic networks provide data essential for assessing whether such swarms signal evolving stress conditions or remain isolated phenomena.
References USGS Earthquake Hazards Program California Geological Survey Fault Activity Map Southern California Earthquake Data Center Catalog