An earthquake swarm designated S20041124.1 occurred 22 km south-southeast of Tres Pinos in San Benito County, California. The sequence began at 08:06 on 23 November 2004 and concluded at 18:37 on 28 November 2004, spanning 130 hours and 31 minutes. During this period, 80 earthquakes were recorded.
The swarm was characterized by low to moderate magnitudes, with the largest event reaching magnitude 4.3 at a depth of 6 km on 24 November 2004 at 02:06:04. Depths throughout the sequence ranged primarily between 0 and 10 km, with the majority clustered around 5 km. Activity peaked within the first 24 hours, featuring multiple events above magnitude 2.0, followed by a gradual decline over subsequent days.
This swarm took place within the California Coast Ranges, a region shaped by ongoing transform tectonics along the San Andreas Fault system. Tres Pinos lies near the Calaveras Fault, a major right-lateral strike-slip structure that accommodates a significant portion of the Pacific-North American plate motion. The local geology consists of Mesozoic Franciscan Complex rocks overlain by Cenozoic sedimentary units, creating a structurally complex zone prone to distributed seismicity.
Historical records indicate that earthquake swarms are recurrent in this area. Since 1 January 2000, three prior swarms have been documented: one each in 2000, 2001, and 2003. These episodes typically involve numerous small-magnitude events without a single dominant mainshock, consistent with the characteristics observed in S20041124.1.
The 2004 swarm did not produce reported surface rupture or significant damage, aligning with the modest energy release of the largest events. Such sequences contribute to strain release along secondary fault strands within the broader San Andreas system and are monitored to refine understanding of fault interactions in central California.
US Geological Survey Earthquake Catalog
California Geological Survey Regional Geologic Maps
Southern California Earthquake Data Center Swarm Database