A seismic swarm designated S20181102.1 occurred 13 km south of Tres Pinos in San Benito County, California. The sequence began at 18:42 UTC on 1 November 2018 and concluded at 15:48 UTC on 5 November 2018, spanning 93 hours and 6 minutes. During this period, 53 earthquakes were recorded.
The swarm initiated with a magnitude 2.1 event at a depth of 2 km. Activity intensified on 2 November, highlighted by a magnitude 4.1 earthquake at 12:58 UTC (depth 6 km), followed closely by a magnitude 3.0 event at 13:01 UTC (depth 5 km) and a magnitude 3.5 at 13:22 UTC (depth 2 km). Subsequent events included multiple magnitude 3+ shocks, such as a 3.5 at 15:27 UTC and a 3.3 at 16:19 UTC, with depths generally ranging between 1 km and 8 km. The sequence tapered with smaller events through 5 November, ending with a magnitude 1.7 at 15:48 UTC (depth 3 km). Depths remained shallow throughout, consistent with activity in the upper crust.
This swarm reflects clustered seismicity typical of the region, where numerous low-magnitude events occur without a dominant mainshock-aftershock pattern. The largest event (M4.1) did not trigger widespread aftershocks exceeding magnitude 3, indicating distributed stress release along local fault structures.
Tres Pinos lies within the central Coast Ranges, adjacent to the San Andreas Fault system. This transform boundary accommodates right-lateral strike-slip motion between the Pacific and North American plates at rates of approximately 35–40 mm per year. The local geology features Franciscan Complex rocks and sedimentary units deformed by faulting. Historical seismicity in San Benito County includes both creeping and locked segments of the San Andreas Fault, with notable activity near the Calaveras and Sargent faults to the north.
Since 2000, the area has experienced 20 documented swarms, occurring in the following years and frequencies: 2000 (1), 2001 (2), 2003 (1), 2004 (1), 2005 (1), 2006 (3), 2007 (2), 2008 (1), 2009 (1), 2011 (3), 2012 (1), 2014 (1), 2017 (1), and 2018 (1). These recurrent swarms underscore ongoing tectonic adjustment in a zone of complex fault interaction.