The San Andreas fault is one of California's most dangerous, and is the state's longest fault. Yet for Southern California, the last big earthquake to strike the southern San Andreas was in 1857, when a magnitude 7.9 earthquake ruptured an astonishing 185 miles between Monterey County and the San Gabriel Mountains near Los Angeles.

It has been quiet since then — too quiet, said Thomas Jordan, director of the Southern California Earthquake Center.

"The springs on the San Andreas system have been wound very, very tight. And the southern San Andreas fault, in particular, looks like it's locked, loaded and ready to go," Jordan said in the opening keynote talk.