Hayward Fault Liquefaction Maps Updated by CGS ​

The California Geological Survey has released updated Seismic Hazard Zone maps along the Hayward Fault corridor, and the results are best described as "confirming what geotechnical engineers have been saying at parties that nobody invites them to for decades." The short version: the Bay Mud is still there, it's still liquefiable, and the Hayward Fault is still the most dangerous urban fault in America. Happy Thursday.

The updated maps incorporate new subsurface data from over 2,000 recently digitized borehole logs and improved liquefaction susceptibility modeling using the Boulanger & Idriss (2014) methodology, which sounds like an indie band but is actually the current gold standard for predicting whether soil will turn into a viscous slurry during an earthquake. The results expand the mapped liquefaction zones in several areas of Alameda and Contra Costa counties, which will absolutely delight property owners and their insurance agents.

What's Different ​

The previous maps, last updated in 2008, relied on coarser subsurface data and older analytical methods. The 2026 update benefits from construction-era geotechnical reports that have been digitized and incorporated into CGS's statewide database — a project that took years and involved scanning thousands of boring logs from filing cabinets that haven't been opened since the Clinton administration.

Key expansions include portions of western Fremont, southern Hayward, and the Oakland Coliseum area, the last of which is liquefiable in multiple senses of the word. The maps also refine lateral spreading potential along creek channels, which is relevant if you've ever wondered why that retaining wall near San Lorenzo Creek looks increasingly enthusiastic about moving downhill.

What This Means for Projects ​

Any new development within the updated Seismic Hazard Zones requires a site-specific geotechnical investigation per the Seismic Hazards Mapping Act. This was already true for most of these areas, but the expanded zones now capture parcels that previously sat just outside the mapped boundaries — parcels whose owners are about to discover what a geotechnical investigation costs. (Spoiler: more than they think, less than the foundation repair they'd need without one.)

For existing infrastructure, the maps provide updated baseline data for seismic vulnerability assessments, retrofit prioritization, and that most beloved of engineering activities: explaining to a project manager why the foundation costs just doubled.

The updated maps are available on the CGS EQ Zapp application.