Background. Several contrasting patterns of long-term behavior can be recognized among the numerous major faults in southern California that are known to be currently active on the basis of geologic, geodetic, and seismologic data. Past large-scale displacements represent cumulative slip along some of the faults during long spans of geologic time, and along others during one or more distinctly shorter spans. Moreover, both rates and senses of movement appear to have changed with time along many of the faults. Such contrasting histories, thus far most effectively traced over the last 25 million years of Cenozoic time, may provide some useful clues concerning future movements that can be expected along these major breaks in the earth's crust.