California's 'Big One' Could Be Caused by 2 Faults Combining

When we think about building-leveling earthquakes in California, it can seem like the San Andreas fault is the only game in town. Heck, The Rock even made a movie about it. But a study published last week in Science Advances finds the San Andreas could have an equally dangerous partner in creating the state's next catastrophic quake. The Los Angeles Times reports geophysics professor Julian Lozos, while trying to solve the mystery of the earthquake of 1812, determined that it's possible the San Andreas fault combined with the San Jacinto fault to cause a magnitude-7.5 earthquake—and could do so again. According to a press release, the 1812 earthquake was one of the biggest in California's history and now appears to have been caused when a rupture in the San Jacinto spread to the San Andreas.

"People shouldn't just be thinking about the San Andreas fault," Lozos says in the press release. The lesser-known San Jacinto could actually be a more serious "seismic hazard" because it runs directly under a number of population centers east of Los Angeles. If it combined with the San Andreas to create a "Big One"—as evidence shows it has in the past—the resulting earthquake could kill more than 1,000 people while destroying buildings and starting fires, what the Times calls a "grim seismic scenario." Lozos says it's important to understand how one fault rupturing can trigger ruptures in others. A recent report taking similar multi-fault thinking into account raised California's odds for having a magnitude-8 or larger earthquake by 2045 from 4.7% to 7%. (More earthquake stories.)