The Hayward Fault Zone is a geologic fault zone capable of generating destructive earthquakes. This fault is about 74 mi (119 km) long, situated mainly along the western base of the hills on the east side of San Francisco Bay. It runs through densely populated areas, including Richmond, El Cerrito, Berkeley, Oakland, San Leandro, Castro Valley, Hayward, Union City, Fremont, and San Jose.
The Hayward Fault is parallel to the San Andreas Fault, which lies offshore and through the San Francisco Peninsula. To the east of the Hayward lies the Calaveras Fault. In 2007, the Hayward Fault was discovered to merge with the Calaveras Fault east of San Jose at a depth of 4 miles (6.4 km), with the potential of creating earthquakes much larger than previously anticipated. Some geologists have suggested that the Southern Calaveras should be renamed as the Southern Hayward.
North of San Pablo Bay is the Rodgers Creek Fault, which was shown in 2016 to be linked with the Hayward Fault under San Pablo Bay to form a combined Hayward-Rodgers Creek Fault that is 118 miles (190 km) long, stretching from north of Healdsburg through Santa Rosa down to Alum Rock in San Jose. Another fault further north, the Maacama Fault, is also considered to be part of the "Hayward Fault subsystem".
While the San Andreas Fault is the principal transform boundary between the Pacific Plate and the North American Plate, the Hayward-Rodgers Creek Fault takes up its share of the overall displacement of the two plates.