California Earthquakes and Tsunamis Could be Looming Threats

With the action-packed blockbuster San Andreas having just debuted on Friday, people are now focusing their attention on the possible dangers that could arise from this major fault line. However, new research says that very large earthquakes - and even tsunamis - from several major faults that lie offshore, could be looming threats and surprise southern California residents.

The fault-ridden undersea landscape off of Southern California and northern Baja California is little understood, but researchers have now reported worrisome findings in the Journal of Geophysical Research: Earth Surface that this region has the potential for magnitude 7.9 to 8.0 earthquakes.

That's because, according to the new study, vertical fault zones lurking there have lifted the seafloor in the past, and if they did so again, powerful quakes could send tsunamis rushing toward cities like San Diego and Los Angeles.

"We're dealing with continental collision," geologist Mark Legg of Legg Geophysical in California said in a statement. "That's fundamental. That's why we have this mess of a complicated logjam."

This so-called logjam refers a region called the California Continental Borderland, where there are a number of faults jammed together, created by the tectonic pileup of the Pacific plate and North American plate. These blocks are wedged together all the way from the San Andreas Fault on the east, to the edge of the continental shelf on the west, from 150 to 200 kilometers (90 to 125 miles) offshore. As the Pacific plate slides northwest, away from California, the plates get squeezed and rotated.

While this process wouldn't result in an epic tsunami that plays out as it does in the box-office movie, completely drowning portions of the state, a hazard does exist and deserves closer scrutiny, researchers find. (Scroll to read on...)

To better understand what we may be dealing with, Legg and his colleagues 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 region's largest faults: the Santa Cruz-Catalina Ridge Fault and the Ferrelo Fault. What they found were signs that both faults have experienced thrust faulting - an upward movement of one side of the fault - in the past.

In some areas there is evidence of past vertical movement as much as 10 feet, they report - displacement that, if it were to happen under the sea, could result in destructive tsunamis.

"Such large faults could even have the potential of a magnitude 8 quake," added geologist Christopher Sorlien of the University of California at Santa Barbara, who was not involved in the study.

Given these looming threats, it's surprising then that the California continental shelf is one of the least well mapped and understood continental shelves in the country.

"We've got high resolution maps of the surface of Mars," Legg said, "yet we still don't have decent bathymetry for our own backyard," attributing the lack of data to research budget cuts.

Whereas most continental shelves are about twice as wide and inactive, those in southern California are very narrow and dominated by active faults and tectonics. And as Southern California's pile-up continues, the plate movements that build up seismic stress on the San Andreas are also putting stress on the long Santa Cruz-Catalina Ridge and Ferrelo Faults. Thus, there is no reason to believe that those faults and others in the Borderlands can't rupture in the same manner as the San Andreas, according to Legg.

Moving forward, researchers hope to better understand the undersea landscape of southern California and forecast possible natural disasters and how to prevent them in the future.

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