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A fault line discovered in the 90s is rattling nerves in Southern California, where millions have been living on shaky ground.

The 25-mile Puente Hills thrust fault, which slices below Los Angeles neighborhoods, downtown skyscrapers, bridges and freeways, could cause catastrophic damage.

A week ago today, a magnitude 5.1 earthquake on the fault broke water mains, cracked foundations, and triggered a rock slide. Experts say a 7.5 would be devastating -- and if the Big One struck, it could kill as many as 18,000 people and displace 750,000.

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That’s because much of Los Angeles’ infrastructure isn’t earthquake-proof. Bridges aren't built to modern earthquake codes and hundreds of buildings are made of vulnerable concrete.

Retrofitting the city’s structures would be a time-consuming and costly endeavor. It took three years and a quarter billion dollars to retrofit LA's Hall of Justice -- and it would cost $400 million to shore up 11 critical bridges.

Los Angeles Mayor Eric Garcetti, who has encouraged residents to visit the disaster awareness website www.ReadyLA.org, fears the Big One could be the biggest disaster in U.S. history.

"We have to make sure we minimize that by seeing fewer structures come down, minimize the damages, and most importantly the loss of life," he said.

Although the next quake is impossible to predict, experts agree that the big one is certain to hit.

“Southern California is the most likely source of … an earthquake of 7.8 or greater in the United States at this point,” said U.S. Geological Survey seismologist Lucy Jones.

Known as the “Earthquake Lady,” Jones is on a crusade, making multiple media appearances and, most recently, setting up a Twitter account, to ensure residents hear her message: be prepared.

"What we are trying to prevent is the end of Los Angeles," she said.