Experts identify ‘creep’ on southern Concord Fault
CONCORD, CA (Aug. 21, 2025) — The threat of a 6.7 magnitude earthquake now extends to a larger swath of the city, after a new geological survey showed unexpected movement on the southern end of the Concord Fault.
“The Concord Fault is creeping along its entire length, not just the northern half, and the decadal average creep rate is roughly consistent along the length of the fault,” said Austin Elliott of the U.S. Geological Survey (USGS) Earthquake Science Center.
The researchers found “shallow creep” – moving about a tenth of an inch a year – about 4.4 miles farther south in Ygnacio Valley neighborhoods than previously thought. In the July 2 Seismological Research Letter, they documented “right-laterally deflected concrete curbs and sidewalk slabs” in the Madigan Avenue area of southeast Concord and northeast Walnut Creek.
Any change in the fault line could affect emergency warning systems as well as property values, notes Andrew Alden, a local geologist and former USGS employee.
“The findings mean the state will need to enlarge its official warning status to include a zone some 4½ miles long, right under a string of hundreds of single-family homes,” he wrote on oaklandgeology.com.
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Tracking the fault
The Concord Fault is a major branch of the Pacific-North America transform plate in Northern California. It runs 12½ miles from North Gate Road in Walnut Creek through Concord, roughly following the course of the 1937 Contra Costa Canal.
The southern end is defined as south of downtown Concord, with the northern part running from BART’s Concord maintenance yard north to Suisun Bay. From there, it continues northward as the Green Valley Fault.
According to the report, the Concord Fault is proportionally the most urbanized fault in the region – with nearly 100% covered by buildings or graded land.
“The Concord Fault is completely occupied by human structures, whereas the more familiar Hayward Fault has a fair share of vacant land that isn’t harmed by either creep or quakes,” Alden said.
Scientists have monitored creep in the northern part of the fault for decades, but Elliott and his team decided to investigate the southern area after hearing that the region’s creep was observed from space. Using a 0.05 arcsec precision robotic total station, they recorded changes to curbs and sidewalks on 28 of 30 streets.
“The fault has moved the curbs as much as a handspan (18 centimeters) since the time they were installed, up to 60 years ago,” Alden said. “The team figured that the fault is moving about 3 millimeters per year here, which fits with estimates for the northern part.”
What’s next
Given the survey information, the state could establish a new earthquake fault zone under the Alquist-Priolo Fault Zoning Act. According to Alden, that means builders in the area would have to hire geo-professionals and home sellers would need to inform buyers of the house’s geological status.
Colleen Awad, Concord’s Community Relations manager, said the city is awaiting final word from USGS – a process that could take several months. In the meantime, officials from Concord and Walnut Creek are working to ensure a coordinated response.
“Once the maps are officially adopted, the city will make the appropriate changes to the General Plan and, as necessary, issue public notifications and work with residents and businesses on any steps necessary toward compliance with state law to protect safety,” Awad said.
Bev Britton
Bev Britton graduated with a degree in journalism from the University of North Dakota and moved to the Bay Area with her soon-to-be husband Jim in 1986. She was features editor at the Contra Costa Times in Walnut Creek before becoming managing editor of the Contra Costa Sun in Lafayette in 1995. She retired from newsrooms in 2001, but an ad for the Clayton Pioneer drew her back in. The family moved to Lake Wildwood in the Gold Country a few years ago - but working at the Pioneer keeps her in touch with her old neighborhoods in Concord and Clayton.