COMPARISON OF THE GROUND-MOTION ATTENUATION DURING THE ۱۹۹۰ MANJIL, IRAN, AND THE ۱۹۸۹ LOMA PRIETA, USA, EARTHQUAKES

The observed peak ground acceleration (PGA) in the vertical and horizontal directions for the destructive earthquakes of October ۱۷, ۱۹۸۹, in California (Loma Prieta earthquake) and June ۲۱, ۱۹۹۰, in northern Iran (Manjil earthquake) are studied. The observation distances extend to ۱۰۰ and ۲۰۰ km for Loma Prieta and Manjil, respectively. The two data sets are indepen-dently subjected to nonlinear regression analysis in terms of distance and magnitude. Empiri-cal evaluation of the regression coefficients enables us to predict peak ground acceleration at points of interest for which no observation exists. In addition, such a study helps us to better understand regional differences in the behavior of strong ground motion in the near-source regions. Based on a preliminary analysis of the limited data available for the Manjil earthquake. it appears that the projected peak accelerations at the source are considerably higher (at least by ۵۰%) than those reported for the Loma Prieta earthquake. Alternatively, ground-motion observations during the Manjil earthquake imply higher attenuation rate for the high frequency motion in northern Iran than in the San Francisco Bay area