An earthquake swarm has been in progress along the northern segment of the Calaveras fault, about 3 to 4 km (1.8 to 2.5 miles) southeast of the center of San Ramon, California, since November 9. Data from the United States Geological Survey (USGS) show at least 40 recorded events ranging in magnitude from M0.6 to M3.8 at depths between 5 and 10 km (3 to 6 miles).
The strongest earthquake, M3.8 at 09:38 local time (17:38 UTC) on November 9, was followed within minutes by several M3.0 and M2.5 events.
The clustering pattern, absence of a clear mainshock, and tight spatial concentration indicate a typical fault-segment swarm rather than a standard aftershock sequence.
No damage or injuries have been reported by local authorities or through the USGS Did You Feel It network.
The main sequence consisted of repeated small-magnitude earthquakes concentrated within a narrow zone about 4 km (2.5 miles) long. Depths ranged between 7 and 9 km (4 to 6 miles), corresponding to the upper crustal section of the Calaveras fault system.
The largest shocks occurred in rapid succession: M3.8 at 09:38 LT (17:38 UTC) at 9.2 km (5.7 miles) depth, followed by M3.2 and M3.0 at 09:42 LT (17:42 UTC) and another M3.0 at 11:50 LT (19:50 UTC), all at depths near 9 km (5.6 miles). Most subsequent tremors were between M1.0 and M2.0 and decreased gradually through November 10.
The Calaveras fault is a major right-lateral strike-slip fault forming the eastern branch of the San Andreas fault system. It extends for about 190 km (118 miles) from Hollister northward through the East Bay, where it connects with the Hayward fault near Dublin and Danville.
The fault passes beneath densely populated areas of the East Bay, including San Ramon, Dublin, and Danville.
The northern Calaveras fault is characterized by recurrent low-magnitude swarms, often linked to minor slip transients or local changes in fluid pressure at depth.
Similar sequences occurred in 2015, 2017, and 2021, each lasting one to three days with magnitudes up to around M4.0. Such swarms are considered part of the region’s normal tectonic adjustment and do not necessarily indicate an imminent larger earthquake. However, geological and seismic studies show that the Calaveras fault is capable of generating an event in the M6.5 to M7.0 range over long-term timescales.
USGS and the California Geological Survey continue to monitor the sequence through the Northern California Seismic System. As of November 10, no anomalous deformation or gas-emission signals have been detected along the fault segment.
Smaller aftershocks below M2 are expected to continue intermittently.
