Earthquakes can not be predicted.
Geologists can, however, calculate the probability of future earthquakes based on past activity.
They know that very large earthquakes have happened along the southern part of the San Andreas Fault about every 150 years for the past 1,500 years or so.
A large quake hit that area in 1857, so they believe another may be due there in the next several decades.
Some have estimated the probability of a major earthquake in San Francisco at 67 percent over the next 30 years.
Earthquakes can occur almost anywhere in the United States, but as this map shows, most occur on the West Coast and in Alaska.