Southern California and its seismologists received a wake-up call on June 28, 1992. The
largest earthquake to strike southern California in 40 years occurred near the town of Landers,
located 30 km north of the San Andreas fault. It had a magnitude of 7.5 (M7.5). Three and one-half
hours later, a M6.5 aftershock struck the Big Bear area 40 km (kilometers) to the west of
Landers. An ad hoc working group was rapidly convened in July, 1992, to evaluate how the
Landers-Big Bear earthquake sequence might affect future large earthquakes along major faults
in southern California. In particular, what are the chances of large earthquakes in the next few
years and how do they compare to previous estimates (such as those of the Working Group on
California Earthquake Probabilities -- WGCEP, 1988)? Such an evaluation was made for central
California after the Lorna Prieta earthquake of 1989 (WGCEP, 1990). The charge to the Landers
ad hoc working group included analyzing the seismicity for the last several years in southern
California and the new paleoseismic, geologic, and geodetic data recently available for southern
California. To inform the public about the potential hazard of plausible earthquakes, the working
group was also asked to map the predicted severity of ground shaking for such earthquakes
compared to that from the Landers earthquake