While scientists say the new movie San Andreas is over the top when it comes to its exaggerated portrayal of a disaster hitting California, they emphasize that a very real earthquake and tsunami risk lies off the coast of Southern California.

A real quake-tsunami wouldn't play out as it does in the box-office disaster movie—portions of the state wouldn't be utterly drowned as the screenplay has it—but a hazard does exist and deserves closer scrutiny, a new study suggests.

"That has not been looked at carefully when it comes to the potential for large earthquakes and tsunamis from offshore faults," says lead author and geologist Mark Legg, who heads a consulting firm in Southern California.

"There are many active faults offshore southern California, which could produce greater than magnitude 7 quakes and tsunamis," he says.

Although California's San Andreas Fault gets all the attention and research, it's the little-known but fault-ridden undersea landscape off the coast of Southern California and Mexico's northern Baja California that have Legg and his colleagues worried.

The new study suggests that vertical fault zones lurking there have lifted the seafloor in the past, and if they did so again, a possible 7.0 to 8.0 magnitude quake could send tsunamis rushing toward San Diego and Los Angeles.

In a region called the California Continental Borderland, there are a number of faults jammed together, created by the tectonic pileup of the Pacific plate and North American plate, Legg explains.

"We're dealing with continental collision," he says. "That's fundamental. That's why we have this mess of a complicated logjam."

In the study, the researchers analyzed mapping data from a survey in 2010 that covered more than 2,800 miles of fault lines on the ocean floor.

They focused on two of the largest of the region's faults: the Santa Cruz-Catalina Ridge Fault and the Ferrelo Fault.

Both faults show signs of having experienced thrust faulting—an upward movement of one side of the fault—in the past, they say.

In some areas there are signs of past vertical movement as much as 10 feet, they report.

That's the kind of displacement that, if it happens under the sea, can result in tsunamis, they explain.

"There's a smoking gun," Legg says.

"I want people to be prepared for the 'unexpected' large earthquake offshore that may be very dangerous due to the added potential for a local tsunami, which would have less than 30 minutes between earthquake and tsunami arrival on the southern California coast," he says.

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