San Francisco, on October 17, 1989. More than 60,000 fans have packed Candlestick Park for game three of the World Series, between the San Francisco Giants and the Oakland A's.
0:14At four minutes past five, the television broadcast has just begun. Then the stadium starts to move, and the live picture, going out to millions of viewers, cuts to static.
0:26This is the story of the Loma Prieta earthquake. We'll look at the fault behind it, the warnings that came before it, and how a whole region rebuilt around its lessons.
0:42The Fault Next Door
California sits on the boundary between two tectonic plates. The Pacific plate is grinding northwest past the North American plate, at about 2 inches a year.
0:53Most of that motion takes place along the San Andreas fault, which runs for more than 700 miles through California. Around San Francisco Bay, it splits into a family of faults, including the San Andreas, the Hayward and the Calaveras.
1:11Like the plates beneath Japan, these faults do not slide smoothly. They lock together, the rock around them bends for decades or even centuries, and then the stored strain is released all at once.
San Francisco learned that the hard way on April 18, 1906, when a magnitude 7.9 earthquake ripped open the San Andreas fault for nearly 300 miles.
1:39Broken gas lines and water mains turned the shaking into a firestorm that burned for days. About 28,000 buildings were destroyed, and more than 3,000 people died.
1:52North of the city, roads and fences that crossed the fault were torn apart and shifted sideways. Near the town of Olema, the ground on one side of the fault moved about 21 feet past the other.
2:05A scientist named Harry Fielding Reid studied those measurements and worked out why earthquakes happen. The rock on either side of a locked fault bends slowly, storing energy, until it snaps back into place. He called this elastic rebound, and it is still how geologists explain earthquakes.
2:23The city rebuilt quickly, and in 1915 it celebrated with a world's fair. To make room for the fairgrounds, a cove along the northern shore was filled in with sand pumped from the bay, along with some debris, and that new land later became a neighborhood called the Marina District.
2:40The Forecast
By the 1980s, scientists were estimating which parts of the San Andreas were most likely to break next. One of the stretches they flagged was in the southern Santa Cruz Mountains, south of San Francisco.
2:54In 1988, a national panel of earthquake experts gave that segment a 30% chance of a large earthquake within 30 years.
3:05The earthquake came the very next year.
3:14Fifteen Seconds
At four minutes past five, the fault broke about 11 miles below the Santa Cruz Mountains, near Loma Prieta peak. The earthquake measured magnitude 6.9.
3:25The strongest shaking lasted only about 15 seconds, and the rupture never reached the surface. Its seismic waves, however, spread across the whole region, and in some places the ground itself made them worse.
3:40Close to the epicenter, old brick buildings on Santa Cruz's Pacific Garden Mall crumbled into the street, and three people were killed there.
3:51About 60 miles away, in the Marina District, the shaking turned the loose, wet ground beneath the houses almost to liquid. Buildings sank, and some collapsed.
4:03In effect, the land made for the city's celebration of its recovery helped bring buildings down 74 years later. Fires broke out from broken gas lines, four people died in the Marina, and seven buildings collapsed.
4:17The Marina was not the only place at risk. All around the bay, buildings and roads stood on soft mud and fill, and in soft ground, seismic waves slow down and grow larger, like jelly shaking in a bowl. Those sites shook two to five times harder than solid rock nearby.
4:35On the Bay Bridge, a 50 foot section of the upper deck broke loose and fell onto the deck below. One person was killed, and the bridge was closed for about a month.
4:47The worst disaster came in Oakland. The Cypress Street Viaduct was a double-decker freeway, and its upper deck collapsed onto the lower one, onto cars in the evening commute. Forty-one people died there.
5:01Traffic that evening was lighter than usual, because many people had gone home early to watch the World Series, and experts believe that kept the death toll from being far worse.
5:13Rescuers searched the wreckage for days. Four days after the earthquake, after 90 hours beneath the fallen deck, they found a man still alive in his car. He survived the rescue, but died a few weeks later.
5:29Aftermath
In all, 63 people died and more than 3,700 were injured. More than 18,000 homes were damaged, some 12,000 people were left without one, and the damage came to about $6 billion.
5:44At Candlestick Park, the stadium held, and the fans walked out safely. The World Series resumed ten days later, and the A's went on to sweep it.
5:57What Changed
The earthquake also reshaped San Francisco. The damaged Embarcadero Freeway, a wall of concrete along the waterfront, was torn down in 1991, and the city's waterfront opened back up to the bay.
6:11The eastern half of the Bay Bridge was replaced entirely. The new span, built to withstand a major earthquake, opened in 2013, at a cost of about $6.4 billion.
6:25Across California, engineers returned to freeways and bridges built before modern building codes, wrapping old concrete columns in steel and tying their spans together.
6:37California also built an earthquake early warning system, which detects the first, faster waves and sends an alert ahead of the strong shaking. On Loma Prieta's thirtieth anniversary, in 2019, the state began sending those alerts to phones.
6:56The Next One
The plates have not stopped moving. Many scientists consider the Hayward fault the most dangerous in the region, because it runs directly beneath the cities on the east side of the bay, and it last broke in 1868.
7:10You can actually see the Hayward fault move. Parts of it creep along continuously, about a fifth of an inch a year, and in downtown Hayward, old curbs that cross the fault have been bent sideways, bit by bit, over the decades.
7:26Its last large earthquake, in 1868, wrecked almost every building in Hayward and killed about 30 people. Until 1906, it was the earthquake people called the great San Francisco earthquake.
7:40The U.S. Geological Survey puts the chance of at least one magnitude 6.7 or larger earthquake in the Bay Area, between 2014 and 2043, at 72%.
7:55That is why, every October, around the anniversary of Loma Prieta, millions of people practice three simple steps in the Great ShakeOut drill: drop to your hands and knees, take cover under a sturdy table, and hold on until the shaking stops.
8:12So, three things to remember. First, Loma Prieta was a magnitude 6.9 earthquake on the San Andreas fault system, and its strongest shaking lasted only about 15 seconds. Second, the worst damage came far from the fault, in places such as the Marina District and the Cypress freeway, where soft mud and fill made the shaking two to five times stronger. And third, the Bay Area is not finished: the chance of another earthquake of magnitude 6.7 or larger there, between 2014 and 2043, is 72%.
8:21Loma Prieta was not the big one, but it was a reminder. The faults beneath the bay are still storing up strain, about an inch at a time.
