Southern California experiences earthquakes on the San Andreas system
of vertical right-lateral predominantly strike-slip faults and on a se
cond system of faults that includes thrusts, oblique-slip, left-latera
l, and other faults. Pattern recognition and cluster analysis are used
to analyze the catalog of earthquakes with magnitudes greater than or
equal to 5.5 from 1915 to 1994. We use pattern recognition to find a
suite of traits that would characterize 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 or th
e other system is the predominant form of earthquake activity. For the
past 2 decades the second system has been the active one. Small chang
es in the direction of plate movements could account for this phenomen
on. Seismic release on the San Andreas system is preceded by episodes
of activity in the Great Basin or in the Gulf of California. Presumabl
y, these episodes would represent extension in the former region and s
preading and slip on transform faults in the latter.