San Francisco Bay Area — the urban East Bay · Right-lateral strike-slip (creeping + locked)
The Hayward Fault is commonly called the most dangerous fault in the United States — not because it is the biggest, but because it is the single most urbanized active fault in the country. It runs along the western base of the East Bay hills, directly through densely populated Richmond, Berkeley, Oakland, San Leandro, Hayward, and Fremont, with hundreds of homes sitting on the trace and freeways, BART, and gas and water lines crossing it. It last ruptured in 1868, and it is, in USGS words, "due."
The fault bisects the UC Berkeley campus (it runs through California Memorial Stadium) and the cities of the East Bay. The population near the fault grew from about 24,000 in 1868 to more than 2.4 million today; a USGS scenario puts more than 7 million people in the region that would feel strong to very strong shaking, with over 1.5 million workers and more than $100 billion a year in wages exposed. A repeat of 1868 today is projected to cause economic losses exceeding $120 billion. The "most dangerous" label reflects this concentration of people and infrastructure on the fault — its danger is probability multiplied by exposure.
On October 21, 1868, the Hayward Fault ruptured in a M6.8 earthquake (estimates range 6.8–7.0), breaking about 32 km (20 mi) of surface trace from Warm Springs (Fremont) north to San Leandro, and possibly as far as Berkeley, with about 6 feet (2 m) of offset. About 30 people died and strong shaking lasted more than 40 seconds. Before 1906, it was known as the "great San Francisco earthquake." Paleoseismic studies find a large earthquake roughly every 140–160 years — and it has been about 158 years since 1868.
Parts of the Hayward Fault creep steadily at the surface (about 3–6 mm/yr), visibly offsetting curbs, streets, and buildings. But that creep is confined to the topmost ~5 km and accounts for only about one-third of the fault's long-term slip; below that depth the fault is locked and storing strain, and parts — notably the northern section — are locked all the way to the surface. Creep does not relieve the hazard; the long-term slip rate is about 9 ± 2 mm/yr, and most of it accrues in large earthquakes.
In 2016, researchers found that the Hayward Fault connects with the Rodgers Creek Fault beneath San Pablo Bay (Watt et al., 2016), making a single ~140 km system. A rupture of the combined system could reach about M7.2–7.4 — larger than the Hayward alone (modeled at about M6.9). The USGS HayWired scenario models a M7.0 mainshock rupturing about 52 miles. Statewide, UCERF3 estimates about a 72% probability of a M6.7+ earthquake somewhere in California within 30 years.
The Hayward Fault is the reason the East Bay takes seismic retrofitting seriously. Its paleoseismic dates carry some uncertainty (a 2014 radiocarbon recalibration revised the prehistoric timeline), and its probabilities depend on the model used — but the central fact is plain: a major fault under millions of people, past its average recurrence. See the Calaveras Fault, the California page, and our seismic risk lookup.