The Hayward Fault has a 72% chance of a major quake before 2045 and could cut water to 2.4 million people

On October 21, 1868, the Hayward Fault ruptured in a magnitude 7.0 earthquake that caused widespread damage across the young East Bay towns and in San Francisco itself, at the time the most destructive earthquake Northern California had recorded. The U.S. Geological Survey now calculates a 72% chance that a fault inside that same regional network will produce a magnitude 6.7 or larger earthquake before 2045, and it treats the Hayward Fault, running directly beneath some of the East Bay’s densest suburbs, as the strand most likely to be the one that ruptures next. A repeat large enough to sever the pipelines crossing its trace could cut water service to 2.4 million people.

The 1868 Rupture Behind a 2045 Forecast

The Hayward Fault is a strike-slip strand of the San Andreas system, meaning the ground on either side grinds sideways past itself rather than lifting or dropping, and it forms part of a wider network of faults threading the Bay Area that the U.S. Geological Survey models together rather than one at a time. That joint model is where the 72% figure comes from: it is the odds that at least one fault in the network breaks at magnitude 6.7 or larger sometime before 2045, and Hayward is the strand USGS geologists flag most often as the likely source, both for its history and for how directly it runs beneath population.

The fault’s last big rupture was the magnitude 7.0 earthquake of October 21, 1868, and the fault’s own hazard page states a 72% chance of experiencing a magnitude 6.7 or greater earthquake before 2045, more than a century and a half after that last major break.

A Fault Line Under Ten Bay Area Cities and Their Pipelines

The trace runs through Richmond, El Cerrito, Berkeley, Oakland, San Leandro, Castro Valley, Hayward, Union City, Fremont and San Jose, some of the most densely built suburbs in Northern California, and hundreds of homes and other structures sit directly on top of it. Interstate 80, Interstate 880, Interstate 580 and two state highways all cross the fault along its length, as does Bay Area Rapid Transit’s underground and elevated track.

Critical infrastructure crosses the same line: regional gas and water pipelines and electrical transmission lines cross the fault, including segments of the Hetch Hetchy Aqueduct that carries water from the Sierra Nevada to San Francisco, and a major refinery with crude-oil docks sits close enough to the trace that planners model it as a rupture-adjacent hazard. It is those water pipelines that produce the largest single number in any Hayward Fault forecast: a major break could leave 2.4 million people across the Bay Area without running water.

The HayWired Simulation Built to See the Aftermath

To turn those probabilities into something emergency planners could act on, the U.S. Geological Survey led hundreds of scientists, engineers and social scientists through a multi-year modeling project called HayWired, simulating a magnitude 7.0 rupture on the Hayward Fault in enough detail to estimate building-by-building damage, fire spread, and how long different neighborhoods would go without water, power and phone service.

Ken Hudnut, the USGS science adviser for risk reduction who helped lead the project, said “the USGS and its partners have worked together to anticipate the impacts of a hypothetical M7.0 earthquake on the Hayward Fault, before it happens, so that people can use the latest science in their efforts to become even better prepared.”

The scenario’s damage estimates run well past the water system alone: reporting on the project found projected losses topping $100 billion and as many as 800 deaths, with some households going without water for six weeks and others as long as six months. David Schwartz, a USGS earthquake geologist who has studied the fault for decades, said “the Hayward Fault is probably one of the most hazardous faults in the country, and the reason is that it passes through the most highly urbanized part of the Bay Area.”

Fault Creep and the Buildings Sitting on the Line

Unlike a fault that stays locked for centuries and then slips all at once, sections of the Hayward Fault creep continuously, moving a few millimeters a year even between major earthquakes. That slow motion has visibly cracked curbs, offset sidewalks and cracked building foundations in Hayward and Berkeley, most notably running directly beneath the University of California’s Memorial Stadium, which engineers later retrofitted with expansion joints designed to let the fault keep moving without collapsing the structure above it.

Creep releases some strain harmlessly, but USGS scientists say it does not substitute for the sudden slip a large earthquake requires, which is part of why the fault’s 72% probability window closes in 2045 rather than resetting with every small movement recorded along its length.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.