Now researchers propose that four faults have built up enough seismic strain (stored energy) to unleash destructive earthquakes, according to a study published today (Oct. 13) in the Bulletin of the Seismological Society of America.

The quartet includes the Hayward Fault, the Rodgers Creek Fault, the Green Valley Fault and the Calaveras Fault. While all are smaller pieces of California’s San Andreas Fault system, which is more than 800 miles (1,300 kilometers) long, the four faults are a serious threat because they directly underlie cities. [Photo Journal: The Gorgeous San Andreas Fault]

“The Hayward Fault is just right in the heart of where people live, and the most buildings and the most infrastructure,” said Jim Lienkaemper, lead study author and a research geophysicist at the U.S. Geological Survey’s Earthquake Science Center in Menlo Park, California. “But it’s not just one fault, it’s the whole shopping basket. If you are in the middle of the Bay Area, you are near a whole lot of faults, and I’m concerned about all of them.”