Not Like 'San Andreas,' But L.A. Faces Real Tsunami Danger

There are some under-appreciated, San Andreas-like faults off Los Angeles that could unleash large quakes and tsunamis. But it's nothing like Hollywood's version.

While scientists are clucking their tongues at the exaggerated disaster portrayed in the new movie "San Andreas," there lurks a very real quake and tsunami danger offshore Southern California, say geologists in a new study.

Several long faults with the potential for magnitude 8.0 quakes and tsunami-making seafloor uplift are within 90 miles of the coast, confirms geophysicist Mark Legg and his colleagues.

"There are many active faults offshore southern California which could produce greater then magnitude 7 quakes and tsunamis," said geologist Mark Legg of Legg Geophysical in Huntington Beach, Calif.

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Evidence for the potentially dangerous nature of the 20-million-year-old faults has been growing for decades as Legg and his colleagues have pieced together data from scores of different research cruises to the area off the coast scientists call the California Continental Borderland.

The latest data is 4,500 miles of new seafloor bathymetry which has been used to work out the lengths of two of the largest faults -- the Santa Cruz-Catalina Ridge Fault and the Ferrelo Fault -- and how they are connected to smaller faults in the Borderland.

Legg and his colleagues reported the new findings about the two faults in a paper accepted for publication in the Journal of Geophysical Research – Earth Surface.

Length matters because a longer fault has more length which can unzip, as happened so recently in Nepal. Figuring out the details of faulting in the Borderland is critical to understanding the earthquake danger.

"The more connected the faults are, the more they can cause larger earthquakes," said Paul Umhoefer, a geologist at Northern Arizona University. "The more detailed data that was gathered in this study is important for judging whether there is an earthquake and tsunami hazard."

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Another important factor in whether the faults can produce tsunamis is what Legg calls transpression. That's when a fault slips sideways like the San Andreas, but binds up and compresses the two sides of the fault together as well. When a fault ruptures, transpression can cause the land -- or seafloor in this case -- to suddenly rise up. That, in turn, can send a tsunami-making pulse up and towards shore.

"It's a rather uncommon tsunami generator," said Umhoefer, but perfectly plausible.

That said, the quakes and tsunamis Legg is talking about are nothing like those seen in the disaster movie. The geology of Southern California doesn't have what it takes to produce megathrust quakes and large ocean-wide tsunamis. That usually requires a subduction zone, which is where one tectonic plate is being forced under another. That does not exist off southern California.

"California does have an active subduction zone north of Mendocino -- Cascadia -- which is capable of magnitude 9 earthquakes and big tsunamis--but that is NOT the San Andreas," said Legg.

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In an earlier presentation for the Seismological Society of America, he calculated that a low-angle fault along the outer edge of the Borderlands -- what's called the Patton Escarpment -- could produce a very big event of magnitude 7.9 to 8.3. But even that would only create a regional tsunami, nothing like what was seen in Sumatra in 2004 or Japan in 2011.

"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," said Legg.

"But I try to reassure the public that earthquakes in southern California are survivable and can be considered 'just another California Adventure!'"

Independent science writer Larry O'Hanlon is also the blog manager and social media coordinator for the American Geophysical Union, which publishes the Journal of Geophysical Research - Earth Surface.

A still from the trailer of the new movie "San Andreas" shows imminent destruction by a massive tsunami.

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