A newly identified strand of the Concord Fault is creeping beneath downtown Concord to Lime Ridge
by Martin Steinpress
A new study reported in July 2025, indicates that an active creeping surface trace (branch) of the Concord Fault runs through suburban east Concord and Walnut Creek along a fault strand not previously recognized. The evidence includes horizontal right-lateral offset of concrete curbs and sidewalk slabs on both sides of every cross street, as shown in the photo.
The newly identified fault strand runs southward from downtown Concord between Oak Grove Road and Lime Ridge. It crosses Treat Boulevard and Ygnacio Valley Road and runs through the Woodlands (where it is projected beneath Valle Verde Elementary School), Diablo Shadows and Northgate neighborhoods.
What does this mean for homeowners in the Ygnacio Valley?
Now that the fault’s active trace is known, the state will establish a new Earthquake Fault Zone (EFZ) as required by the Alquist-Priolo Act. EFZs are regulatory zones surrounding the surface traces of active faults in California. Due to the potential for surface rupture, there are disclosure and investigative requirements for real estate transactions and development projects within EFZ zones.
For example, home sellers within an EFZ must inform buyers of the house’s geological status in their Natural Hazard Disclosure Statement in the home-selling process. In addition, proximity to an active fault increases the likelihood of intense shaking and damage to buildings, so the cost of local California Earthquake Authority homeowner’s earthquake insurance policies may increase.
What are the implications for residents in the greater Bay Area?
The Concord Fault is one of the many right-lateral strike-slip faults of the San Andreas Fault System (which also includes the Hayward and Calaveras faults) that together constitute the Pacific–North America transform plate boundary. Like parts of all these fault branches, the Concord Fault’s long-term slip is partially accommodated by slow creep (aseismic slip), meaning it occurs gradually without a significant earthquake.
However, the Concord Fault also has the potential for an earthquake greater than a magnitude (M) 6.7 (roughly equivalent to the 1989 Loma Prieta Earthquake) and perhaps as large as M8.0 (similar to the 1906 San Francisco Earthquake).
A M5.4 quake occurred in 1955, but the last large earthquake was approximately 400 years ago. Much of Concord and Walnut Creek had a reminder of the fault’s presence on May 30, 2025, when a small M3.2 shook the area.
The Concord Fault is the most urban fault in the East Bay and underlies much critical infrastructure, including refineries, pipelines, highways and railroad bridges. In the event of a large quake, intense shaking could render the north half of the state without fuel; water and electricity could be disrupted; and delta levee failures could cause flooding and impact drinking water quality.
The study was conducted by geologists with the U.S. Geological Survey and California Geological Survey (Elliott and others 2025) and the full publication can be found at https://doi.org/10.1785/0220240454.
Martin Steinpress of Concord is a retired California geologist, hydrogeologist and environmental consultant who has also taught geology at California State University East Bay.
He currently lectures on geology, plate tectonics and earthquakes and leads geologic field trips along the Concord Fault and on Mt. Diablo through the Lifelong Education Program at Mt. Diablo Adult Education.
Registrations is open for his next class on March 17-19 under Personal Enrichment at https://mdae.mdusd.org/programs/lifelongeducation.
Or email, martin@steinpress.com.