Quake calculations

Californians call it “the Big One”—a massive earthquake capable of destroying an entire city if the San Andreas Fault ruptures in the wrong spot. San Francisco saw such an earthquake in 1906 when a 7.9 temblor destroyed much of the city and killed thousands. Geologists cite the 1857 Fort Tejon quake that registered as a magnitude 7.9 as another.

After a 2006 report in Nature, geologists have predicted the southernmost section of California’s San Andreas Fault is the most likely location to generate the next Big One. While earlier reports predicted massive damage in the Los Angeles area from such a temblor, newly published research in Scientific Advances from San Jose State University geologist Kimberly Blisniuk suggests the next Big One might not be so bad.

California’s San Andreas Fault stretches for nearly 800 miles along the Pacific Coast before diving inland and running just 35 miles from Los Angeles. The fault marks a place where the Pacific Plate and the North American plate slip past one another. In Southern California, scientists estimate the two major plates generate about an inch of potential movement every year. Rather than neatly slide by, the plates snag. Decades or even centuries of movement translate into potential energy. Eventually the plates break free, causing an earthquake that releases massively powerful seismic waves.

While the northern and central sections of the San Andreas Fault have generated massive earthquakes since 1857, the southern portion of the fault has not. Scientists estimate the last Big One along that section occurred around 1726, decades before Spanish colonization of California began in earnest. According to Blisniuk, the southern section of the San Andreas Fault should generate a colossal earthquake about once every 215 years. That puts the Big One about 80 years overdue.

In 2008, the U.S. Geological Survey studied a hypothetical scenario in which a 7.8 earthquake occurs with an epicenter near the Salton Sea, sending violent shockwaves into the nearby Los Angeles area. Teaming up with geologists and emergency preparedness officials from across the region, the USGS called the mock earthquake trial the Great Southern California ShakeOut. The model assumed the quake would come from the Banning strand of the San Andreas Fault, a subsection of the formation east of Los Angeles. From there, the east-west orientation of the strand would direct massive amounts of energy through the San Bernardino Valley before being focused into the basin around Los Angeles. At the end of the simulation, the scientists predicted about 1,800 deaths and roughly $200 billion in damages to the region.

But Blisniuk’s research suggests that location may not be quite right. She and her team of researchers looked for so-called beheaded channels: ancient canyons that once bisected the fault before thousands of years of tectonic activity carried the dry riverbeds in opposite directions. Over years of trudging through the desert between Palm Springs, Calif., and Joshua Tree National Park, her team identified a number of stream beds slowly being divided by the tectonic plates slipping past one another.

Blisniuk’s team identified three beheaded canyons in the area by observing terrain features. They confirmed that the canyons were once connected by dating the age of rocks on each side of the divide. From there, the scientists measured just how fast the formations were moving away from one another as the continental plates moved them in opposite directions. According to Blisniuk, the plates at the Mission Creek strand to the north moved at a rate of an inch a year—about 10 times faster than at the Banning Strand. According to Blisniuk, that means the Mission Creek strand is likely to generate the massive earthquake Californians have been waiting for.

With nearly 300 years of pent-up movement ready to be released in one massive earthquake, Blisniuk said she expects the plates to move between 20 and 30 feet when the Big One happens. But the Mission Creek strand’s orientation would help direct more energy to the northwest, leading to a smaller impact on the Los Angeles area.

“That’s a significant reduction in risk for L.A. if this is true,” seismologist Lucy Jones, who spearheaded the 2008 ShakeOut, told the Los Angeles Times. “This is a piece in an ongoing debate and not yet completely resolved—probably won’t be, until we have the earthquake.”