A Bay Area fault that could produce a major earthquake is not where scientists thought it was

San Francisco Chronicle​

12-07-2025

The Bay Area's Concord Fault — which is capable of unleashing a major earthquake — isn't where scientists thought it was.
The fault runs for about 20 miles through Walnut Creek and Concord, from North Gate Road near Mount Diablo north to Suisun Bay. A previously unknown 4.4-mile stretch, or strand, of the fault is actively moving about a tenth of an inch per year as it runs beneath residential neighborhoods in the Ygnacio Valley.
'Many of the people we have spoken to have noted their houses and yards being deformed, water lines being broken and other effects of the creep,' Jessie Vermeer, a geologist with the U.S. Geological Survey, said by email.
Vermeer and colleagues reported the new path in a study published July 2. The scientists documented that the southern half of the Concord Fault is one-tenth to three-tenths of a mile west of where researchers previously mapped it.
'By identifying the new strand of the Concord Fault we can use it to refine our earthquake magnitude and timing calculations,' Vermeer said.
The California Geological Survey could classify the new strand as an active fault zone, a designation that affects construction of new buildings, property values and real estate transactions. The findings could also help guide preparations ahead of big quakes.
'Cities, utility companies with underground lines, road repair agencies (and) even homeowners, can understand better where to expect damage, to plan for it financially, or to try and mitigate it altogether,' author Austin Elliott, who led the research as a geologist with the U.S. Geological Survey, said by email.
The Concord Fault can produce earthquakes of 6.7 magnitude or higher, according to the U.S. Geological Survey. A magnitude 5.4 quake on the fault caused widespread damage across the Bay Area in 1955.
The southern portion of the Concord Fault was previously mapped along the edge of Lime Ridge, which extends from the southern edge of Concord to Mount Diablo. The newly identified strand runs beneath residential neighborhoods; it even runs right through Valle Verde Elementary School in Walnut Creek.
In the study, the researchers describe slow and steady movement of tectonic plates, known as creeping, along the newly defined southern segment; this behavior had previously been reported only on the northern half of the Concord Fault.
As the Concord Fault creeps, it can deform structures, break water lines and cause other damage for people living along the fault line, explained Andrew Alden, an Oakland-based geologist and writer, who wasn't part of the new research.
'Having a precise location (for the fault) is really important for emergency response,' Alden said.
The analysis 'is important for understanding the seismic hazards that the fault poses,' Wendy Bohon, branch chief of seismic hazards and earthquake engineering with the California Geological Survey, said by email.
'The more we understand the seismic hazard of an area, the better job we can do at decreasing the seismic risk and increasing the resilience of California communities,' said Bohon, who wasn't part of the new study.
Scientists have known since the 1970s that the northern half of the Concord fault is steadily creeping. This northern section stretches from Concord, alongside Mount Diablo High School and Buchanan Field Airport, through Acme Landfill in Martinez.
The southern half of the fault, however, hadn't been conclusively mapped. In the new study, the authors defined the trace of the southern Concord Fault by identifying and measuring offsets in curbs and sidewalks in the area.
'Some faults break quickly, causing large earthquakes, and some move slowly, creeping along without causing shaking. Other faults do a combination of these two things,' Bohon said.
The authors say that the new findings raise the question of whether there are other active strands of the Concord Fault.