The big bend region of the southern San Andreas fault consists of two great asperities that rupture infrequently in great earthquakes. The eastern knot near San Gorgonio Pass, which has not ruptured historically in a large event, is the main locus of plate motion, appears to break in great events every few hundred years, and is more advanced in the cycle of strain accumulation than the western knot. The large size of these asperities results in the properties of these earthquakes being nearly invariant over many cycles of great shocks. An unusual sequence of moderate-size shocks within the eastern knot from 1940 to 1948 is an example of the type of precursory phenomenon that might precede a large earthquake.
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