So we play the odds. For every spot in southern California, we add up the number of times, on average , it should be shaken by all the faults that can affect it. The result is this map from a major new analysis by scientists of the Southern California Earthquake Center . It shows the average number of times each century that sites in southern California will be shaken with a force of at least 20 percent the force of gravity. This does not mean that each century will have exactly this many earthquakes, just as an annual rainfall of 12 inches does not mean exactly 12 inches of rain each year.
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Individual copies can be purchased from the Seismological Society of America or the Southern California Earthquake Center. The map below presents the hazard analysis in terms of average rates of earthquake shaking. We could have expressed the same information in terms of probabilities of earthquake shaking. For instance, a place that averages one incident of severe shaking each century has a 1 percent probability of shaking in any one year, a 26 percent probability in any 30 years, and a 90 percent probability in any 150 years. This analysis predicts that southern California should experience a magnitude 7.0 or greater earthquake about seven times each century.