At 5:04 p.m. on 17 October 1989, a magnitude 6.9 earthquake struck the Santa Cruz Mountains of central California, about 16 km northeast of Santa Cruz on a segment of the San Andreas Fault, at a depth of roughly 18 km. The rupture was felt across the wider San Francisco Bay Area, some 100 km to the north.
The quake killed 63 people and injured more than 3,750, according to USGS figures. The single deadliest event was the collapse of a 1.25-km elevated section of the Cypress Street Viaduct on Interstate 880 in Oakland, where the upper deck pancaked onto the lower one, killing 42 people. A 15-metre section of the upper deck of the San Francisco–Oakland Bay Bridge also fell onto the deck below, killing one driver.
The earthquake struck minutes before Game 3 of the 1989 World Series was due to start at Candlestick Park, between the San Francisco Giants and the Oakland Athletics, and was broadcast live to a national television audience — giving it the nickname 'the World Series earthquake.' The game was postponed for ten days.
Total damage was estimated at around $6 billion (1989 dollars), making it one of the costliest US earthquakes at the time. USGS studies of Loma Prieta's damage pattern — including severe destruction in San Francisco's Marina District, built on filled bay mud that amplified the shaking — became a key case study in seismic building codes, and led directly to the eventual demolition and replacement of the Cypress Viaduct and the retrofit of the Bay Bridge's eastern span.