An earthquake of magnitude 6.9 struck the San Francisco Bay area on October 17, 1989, at 5.04 in the afternoon, and it killed 63 people. It was centred near Loma Prieta in the Santa Cruz Mountains, on the San Andreas fault, and the shaking lasted about fifteen seconds. The worst single failure was the Cypress Street viaduct in Oakland, a double-deck highway whose upper level dropped onto the lower, crushing the cars beneath and killing 42 people.
It happened on television. The third game of the World Series was about to start at Candlestick Park in San Francisco, between the two Bay Area teams, and the cameras were rolling when the ground moved, so millions of people saw the shaking as it happened, the announcers going silent, the stands swaying, and the picture holding through the aftershocks. The game was called off, and the series resumed ten days later.
The damage ran wide. A section of the San Francisco-Oakland Bay Bridge dropped and left a gap, the Marina district in San Francisco, built on loose fill, collapsed in places and burned, and the cities around the bay were cut off from each other for days. The toll of 63 dead was low for the size of the quake, and the engineers put it down to the building codes that California had tightened after earlier earthquakes, which is why the same shaking that levelled parts of the Bay Area killed fewer people than it would have a generation before.
The quake changed the planning. The Cypress viaduct and the Bay Bridge were rebuilt to stronger standards, and the lessons of Loma Prieta fed into the retrofitting that followed, and into the design of the replacements. It is remembered now as the earthquake that was watched live, and as the test that the region's building codes, written for exactly such a day, mostly passed.