The Big One
The San Andreas Fault, the boundary between the North American and Pacific tectonic plates, runs 800 miles (1,300 kilometers) along the California coast and extends 10 miles (16 kilometers) down into the Earth. It is like a master fault in an intricate network of smaller faults that branch from it and join it.
Most of the faults in the San Francisco Bay Area are so-called strike-slip faults. The 1989 quake happened as rocks 11 miles (18 kilometers) below ground abruptly slipped as much as seven feet (two meters). The rupture extended more than 22 miles (35 kilometers) along the fault, in both directions.
In California, where much of the earthquake talk focuses on the "Big One," many people breathed a little easier after the 1989 quake, thinking they had survived that dreaded event.
But the 1989 quake pales in comparison with the 1906 quake that struck San Francisco with a magnitude of 7.9 and destroyed most of the city. Having a quake on one fault also does not diminish the likelihood of another temblor striking on a different fault.
Experts say the "Big One" is not necessarily going to do the most damage. In bigger earthquakes, shaking merely lasts longer and is felt over a larger area. Smaller earthquakes, on the other hand, have higher-frequency energy (or oscillate more rapidly), which can be particularly harmful to residential homes.
"It's not the infrequent 8-point earthquake that we have to worry about, it's the much more frequent 6.5 quake that we should be concerned with," Uhrhammer said. "They can cause as much damage per square mile as a magnitude 8."
While the epicenter of the 1989 quake was in a sparsely populated area, the next big quake is likely to be centered in a more populated area. A U.S. Geological Survey report last year predicted a 62 percent chance that a 6.7 magnitude or larger quake will strike in the Bay Area in the next 30 years. At particular risk is the Hayward Fault line that runs through the heart of the metropolitan area.
"It will hit San Francisco and the East Bay equally hard," Uhrhammer said. "It will severely impact the whole region."
Soft Soil
Most of the large structures in the area San Francisco Bay Area were earthquake proof in 1989. Even the Bay Bridge, where a section of the upper deck broke and fell onto the lower deck, behaved as it was designed.
The section that came down was located in between double columns. The bolts there were designed to snap in the event an earthquake produced more shaking than the bridge could handle. "If the bridge had broken at a single column, it may have dominoed the whole thing," Uhrhammer said.
The Cypress Freeway, on the other hand, was known to be a weak structure because of the soft soil it was sitting on. As the seismic waves rolled onto the ground like waves breaking on a beach, the structural columns of the freeway simply buckled, and the road decks totally collapsed.
Ironically, city officials had planned to take the Cypress Freeway down in the early 1990s. The quake beat them to it. Since 1989 many of the freeway overpasses around the region have been retrofitted and earthquake-proofed.
But what to do about, for example, residential homes that were built on soft soil vulnerable to the shaking of earthquakes? Building codes were substantially improved in 1975, but much of San Francisco was built in the early 1900s.
"What you have now is a lot of older structures on top of poor soil, which are not up to modern code, and there's no way, short of razing whole blocks of the city, to fix this," said Jack Boatwright, a geophysicist with the U.S. Geological Survey in Menlo Park, south of San Francisco.
It would be impossible to reengineer the landfill under the Marina District, for example. (Some of that landfill contains debris and rubble from the devastating 1906 earthquake.)
The shaking in the 1989 quake caused some of the ground underneath the Marina to liquefy. As a result, many structures lost their foundation and sank into the ground. Fires erupted because of broken gas lines.
"What struck me about the destruction of the Loma Prieta earthquake was the vulnerability of the urban environment," Boatwright said. "It gave us a taste of what might happen if there's a quake closer to the city."
Don't Miss a Discovery
Sign up for the free Inside National Geographic newsletter. Every two weeks we'll send you our top news stories by e-mail.
For more earthquake stories, scroll to bottom.
|
SOURCES AND RELATED WEB SITES
|
