Southern California experiences earthquakes on the San Andreas system of vertical right-lateral predominantly strike-slip faults and on a second system of faults that includes thrusts, oblique-slip, left-lateral, and other faults. Pattern recognition and cluster analysis are used to analyze the catalog of earthquakes with magnitude ≥5.5 from 1915 to 1994. We use pattern recognition to find a suite of traits that would charcterize each of these two systems and distinguish them from each other. Both pattern recognition and cluster analysis show that epochs of seismic release occur in which one of the other system is the predominant form of earthquake activity. For the past 2 decades the second system has been the active one. Small changes in the direction of plate movements could account for this phenomenon. Seismic release on the San Andreas system is preceded by episodes of activity in the Great Basin or in the Gulf of California. Presumably, these episodes would represent extension in the former region and spreading and slip on transform faults in the latter. ¿ American Geophysical Union 1995