At least three major faults in the San Francisco Bay Area seem to be stuck—and when they come unstuck, they could wreak major havoc, NBC News reports. There's an almost 70% chance that one of four "locked" sections of the Hayward, Rodgers Creek, and Green Valley faults could rupture over the next 30 years, says Jim Lienkaemper, the lead author of a study published in the Bulletin of the Seismological Society of America, with the Hayward Fault currently causing the most concern because of the population and infrastructure in its proximity. "A large earthquake on the Hayward Fault would affect the entire San Francisco Bay Area—over 5 million people," an earth sciences professor at Stanford University tells NBC. Sudden ruptures on the other faults would have similarly catastrophic effects, including possibly cutting off drinking water to the area.
When everything's working as it should, the North American Plate in California pushes past the Pacific Plate without much hubbub; in fact, the Pacific Plate's 38mm-per-year movement means that LA could, in millions of years, end up crammed next to San Francisco. But "fault creep"—the movement of the fault segments—sometimes stalls, and "stress will build until it is released by an earthquake," Phys.org reports; three of the fault sections discussed in this study are seeing less than 1mm of creep each year. Experts hope this study will motivate local governments and businesses to earthquake-proof their buildings, though one professor at the University of California–Santa Barbara worries that "within the next 30 years" doesn't suggest the urgency that might be warranted, NBC notes. "What they need to think about is that it could happen tomorrow or a week from tomorrow. … Inevitably there will be an earthquake," he says. (The East Coast may be due for a big temblor soon, too.)