The Hayward Fault is a right-lateral strike-slip fault running about 120 kilometres along the eastern side of San Francisco Bay, threading through the densely populated cities of Oakland, Hayward, Berkeley and Fremont. It is part of the broader San Andreas system, helping accommodate the sideways motion between the Pacific and North American plates.
Unlike some neighbouring faults, the Hayward creeps slowly and visibly at the surface, bending curbs, cracking building foundations and offsetting a football stadium at Berkeley. Geologists estimate it slips roughly 4 to 9 millimetres per year, though much strain remains locked at depth and is released in sudden large earthquakes.
The fault ruptured in 1868 with an estimated magnitude of about 6.8, an event once called the 'great San Francisco earthquake' before 1906 eclipsed it. Paleoseismic studies show major quakes recur roughly every 140 to 170 years, and because the last was over 150 years ago, the Hayward is widely regarded as one of the most dangerous faults in the United States.