Southern Concord Fault found to be slowly shifting, changing seismic hazard maps for Bay Area suburbs

A newly mapped 7 km (4.3 miles) segment of the Concord Fault, now identified as the Madigan Avenue strand, has been confirmed as actively creeping beneath residential neighborhoods in Concord and Walnut Creek, California.

The Concord Fault is a major branch of the Pacific–North America plate boundary system in Northern California, linking the Bartlett Springs–Green Valley Faults in the north with the Greenville and Calaveras Faults to the south. Like many faults in the Bay Area, it releases some of its long-term motion through slow, steady creep rather than sudden earthquakes.

The newly defined trace, located 170–500 m (558–1 640 feet) west of previously mapped fault lines, represents a major shift in how seismic hazard is understood in the area.

Field surveys and alignment array data show consistent right-lateral aseismic slip at a rate of approximately 2–3 mm/year (0.08–0.12 inches/year) along this entire segment. Observed creep-induced deformation includes displaced curbs, sidewalks, and roads, ranging from 7 to 18 cm (2.8–7.1 inches) across surface widths of 1–10 m (3.3–33 feet), proving decades of ongoing fault motion through densely developed zones.

The study shows that almost the entire length of the Concord Fault runs beneath built infrastructure, setting it apart from other regional faults such as the Hayward Fault.

This remapped active strand, previously unrecognized for most of its extent, follows a linear alignment with the earlier-mapped but short Madigan Avenue trace. Mapping of deformation patterns shows that every road crossing the fault exhibits lateral shear deformation, apart from those highly oblique to the strike of the fault. At Valle Verde Elementary School, the lack of rigid structures made it difficult to detect any signs of creep, even though the site lies within the newly mapped fault corridor.

Creep rates along this southern section are comparable to those previously recorded on the northern Concord Fault, which has been under continuous geodetic observation for decades. Consistent slip rates across the full fault length support the conclusion that aseismic deformation is occurring uniformly, raising the possibility of a slip deficit at depth capable of sustaining large seismic events.

The Concord Fault is estimated to be capable of producing earthquakes up to M6.7, and unlike faults confined to rural or peripheral zones, it transects densely populated neighborhoods.

The California Geological Survey plans to establish a new Earthquake Fault Zone (EFZ) based on this updated trace. This designation will extend 60–90 m (197–295 feet) on either side of the fault and impose new regulatory requirements on development, including mandatory geological investigations and disclosure of the fault’s presence during property transactions.

The surface trace terminates near the base of Mount Diablo, where structural complexity increases and multiple splay faults are suggested by geomorphic features. Although no substantial vertical offset has been observed, minor parallel scarps in Quaternary deposits may suggest additional fault segments south of Arbolado Drive, requiring further monitoring to assess their role in future fault activity.

Historical geodetic records also indicate that the Concord Fault undergoes quasi-periodic creep accelerations at intervals of approximately 3.8 ± 1 years, with the most recent event occurring in 2021.

The relocation of the Concord Fault’s active trace beneath urban neighborhoods, not alongside them, has direct implications for seismic risk assessment and land-use planning in the region. Infrastructure within the revised zone has already sustained damage, and hazard models are now being updated to reflect the new trace location and observed creep behavior.

References:

¹ A Newly Identified Creeping Strand of the Concord Fault, San Francisco Bay Area – Austin Elliott at al. – Seismological Research Letters – July 2025 – https://doi.org/10.1785/0220240454 – OPEN ACCESS

Teo Blašković

I'm a dedicated researcher, journalist, and editor at The Watchers. With over 20 years of experience in the media industry, I specialize in hard science news, focusing on extreme weather, seismic and volcanic activity, space weather, and astronomy, including near-Earth objects and planetary defense strategies. You can reach me at teo /at/ watchers.news.

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