An earthquake swarm is in progress along the Calaveras fault near San Francisco, California

A swarm of earthquakes has been advancing along the northern portion of the Calaveras Fault, about 3 to 4 kilometers (1.8 to 2.5 mi) southeast of the center of San Ramon, California, since November 9. USGS data show at least 40 recorded events ranging in magnitude from M0.6 to M3.8 at depths between 5 and 10 kilometers (3 to 6 miles).

The strongest earthquake, a M3.8 at 09:38 local time (17:38 UTC) on 9 November, was followed within minutes by several M3.0 and M2.5 events.

The clustering pattern, absence of a clear mainshock, and narrow spatial focus suggest a typical fault-sector swarm rather than a standard sequence of aftershocks.

No damages or injuries have been reported by local authorities or through the USGS Did You Feel It network.

The main sequence consists of recurring small-scale earthquakes concentrated within a narrow zone about 4 km (2.5 mi) long. Depths ranged from 7 to 9 km (4 to 6 miles), corresponding to the upper crustal section of the Calaveras Fault System.

The largest impacts occurred in quick succession: M3.8 at 09:38 UTC (17:38 UTC) at a depth of 9.2 km (5.7 mi), followed by M3.2 and M3.0 at 09:42 UTC (17:42 UTC) and another M3.0 at 11:50 UTC (19:50 UTC), all at depths near 9 km (5.6 mi). Most subsequent tremors were between M1.0 and M2.0 and gradually decreased until November 10.

A swarm of earthquakes was recorded along the Calaveras Fault near San Ramon, California, on November 9 and 10, 2025. Image source: TW/SAM, Google

The Calaveras Fault is a major right-sided strike-slip fault that forms the eastern branch of the San Andreas Fault System. It extends about 190 kilometers (118 mi) from Hollister northward across the East Bay, where it connects with the Hayward Fault near Dublin and Danville.

The fault passes under densely populated areas of the East Bay, including San Ramon, Dublin and Danville.

The North Calaveras Fault is characterized by frequent occurrences of low-volume swarms, often associated with transient minor slip or local changes in fluid pressure at depth.

Similar sequences occurred in 2015, 2017, and 2021, each lasting one to three days at magnitudes up to about M4.0. Such swarms are part of the natural tectonic adaptation of the area 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 on long-term timescales.

The USGS and California Geological Survey continue to monitor the sequence through the Northern California seismic system. As of November 10, no abnormal deformation or gas emission signals have been detected along the fault segment.

Smaller aftershocks below M2 are expected to continue intermittently.

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