PEAK ACCELERATIONS FROM THE 1992 LANDERS AND BIG-BEAR, CALIFORNIA, EARTHQUAKES

Peak ground accelerations (PGA) at 181 sites for the Landers M(w) 7.3 earthquake and at 85 sites for the Big Bear M(w) 6.2 aftershock are compared to the Joyner and Boore (1988) (JB) attenuation relationships and examined for spatial variations. This study finds the following five results. First, at the 95% confidence level, the mean of PGA values for both earthquakes is significantly higher than the JB median by a factor near 2. Second, for distances greater than 70 km, the observations from the Landers earthquake indicate that attenuation of ground motion with distance is less than that predicted by geometric spreading and by JB. Third, the northward propagation of the Landers rupture significantly reduces PGA values recorded at strong-motion stations south of the epicenter. Fourth, stations in the Los Angeles basin and the Coachella Valley recorded higher PGA values than expected during both events. Finally, site geology, using the site classification scheme of JB, generally does not have a statistically significant effect on PGA, except in the distance range from 60 to 100 km for the Landers earthquake. The observed difference between soil- and rock-site PGA's between 60 and 100 km for the Landers data set may be a combination of higher values of PGA recorded at soil sites in basins, an S-wave radiation maximum in the San Bernardino area, and directivity effects.