It's not an earthquake.
A new report by the USGS says what we have to worry about is a massive flood throughout the entire state, not just an earthquake that might affect only parts of the state.
"If you'd lived in California for 150 years, you would have known that floods were California's Other Big One," said Dr. Lucy Jones of the USGS. "Because they were so bad in the 19th Century, we built a great flood control system, and we've managed to remove the moderate floods. Most people growing up in California, don't think of floods as an issue.
"When we did a study and compared the type of flood that happens every 100 or 200 years with the type of earthquake that happens every 100 or 200 years, we end up concluding that the flood actually costs five times as much as the earthquake."
About $300 billion in damage.
It happened in the late 1860s, and it could happen again, Jones said. In the video above, she takes a look at the costs California would bear.
"The big issue here is the potential disruption to our economy," Jones said. "If we lose one-third of the taxable land in California this time, that's a huge financial hit to all of our systems."