for National Geographic News
It was shortly after 5 p.m. on October 17, 1989. The university library in downtown San Francisco where I was sitting had begun to clear out of students ahead of game three of the World Series between the San Francisco Giants and the Oakland Athletic's, dubbed the "Battle of the Bay."
That day, however, the rival cities would not be battling each other but a common foe. "Earthquake!" someone shouted out as soon as the tremors began. At first the shaking was rapid, vibrating. Then, great waves seemed to roll over us.
I remained frozen in my seathaving moved to California only the year before, I was still an earthquake noviceand watched the bookshelves fall like dominoes. A crack raced up a three-story concrete pillar in front of me.
Fifteen seconds later, it was all over.
One by one, we staggered out onto the street. Businesspeople huddled around a homeless man's radio. The mighty Bay Bridge had collapsed. Later I learned that much of downtown San Franciscositting on landfillhad experienced some of the worst tremors, shaking like a bowl of Jello.
The damage was devastating: A large section of the Cypress Freeway in Oakland collapsed on itself, killing dozens of motorists. In San Francisco the upscale Marina District fared the worst as some of the ground underneath it liquefied, causing buildings to buckle and catch fire. In all, 63 people died, with damage estimated at six billion to ten billion dollars (U.S.).
Remarkably, the epicenter of the 6.9 magnitude quake was nowhere near San Francisco or Oakland, but 60 miles (100 kilometers) southat the Loma Prieta peak, deep in the Santa Cruz Mountains.
So what made the quake so destructive?
One reason is a geological phenomenon known as the Moho bounce. The Moho, named after the Croatian scientist who suggested its existence, is the boundary surface between the Earth's crust and mantle. It can bounce off seismic waves and increase shaking at distances 60 miles away.
But the quake also exposed human folly: Both the Marina District and the Cypress Freeway were built on shaky ground.
"Every place that was damaged was in a marina, on soft soil along the water edge or where old creeks had been filled in," said Robert Uhrhammer, a research seismologist at the Berkeley Seismological Laboratory. "If you overlay the map of damage with a map of soft soils, it's a one-to-one match."
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