Seismic swarm S20171113.1 occurred in central California, centered 17 km south-southeast of Tres Pinos in San Benito County. The sequence began at 19:31 UTC on 13 November 2017 and concluded at 21:22 UTC on 22 November 2017, spanning 217 hours and 50 minutes. During this period, 181 earthquakes were recorded.
The swarm region lies within the California Coast Ranges, a tectonically active zone shaped by the interaction between the Pacific and North American plates along the San Andreas Fault system. Tres Pinos sits near the Calaveras Fault, a major right-lateral strike-slip branch of the San Andreas. This setting produces frequent small-to-moderate earthquakes due to distributed shear and fault creep. Depths of events in the swarm clustered between 3 and 6 km, consistent with shallow crustal faulting typical of the area.
Historical records indicate 18 prior swarms in the same locale since 2000. These occurred in 2000 (1 swarm), 2001 (2), 2003 (1), 2004 (1), 2005 (1), 2006 (3), 2007 (2), 2008 (1), 2009 (1), 2011 (3), 2012 (1), and 2014 (1). Such episodic clusters reflect the region's persistent low-level seismicity driven by regional plate motion.
Examination of the first 100 events reveals a classic swarm pattern without a single dominant mainshock. The initial event reached magnitude 4.5 at 6 km depth. Subsequent activity included multiple events above magnitude 2.0, with notable peaks at magnitudes 3.7, 3.1, 2.8 (three instances), and 2.7. Depths remained shallow, predominantly 4–6 km. Magnitudes generally ranged from 0.6 to 2.8 after the opening shock, showing a gradual decline in both frequency and size over the first two days. The sequence exhibited typical swarm characteristics: numerous events of similar magnitude occurring in rapid succession without clear foreshock-mainshock-aftershock progression.
This activity aligns with known behavior along creeping sections of the Calaveras Fault, where aseismic slip and small earthquakes accommodate strain. No damage or injuries were reported from the swarm.
USGS Earthquake Catalog (events 2000–2017)
California Geological Survey, Regional Fault Maps
USGS San Andreas Fault System Overview (updated 2023)