A STUDY ON THE EMPIRICAL PGA ATTENUATION RELATIONSHIPS IN IRAN

The strong ground motions data that are recorded in 1975 to 2003 in Iran are used to derive the empirical attenuation laws for PGA, as a function of Mw, hypocentral distances, and a constant parameter representing the site conditions (Ci). The method of two steps regressions is proposed. The site soil conditions are considered in the attenuation model, for which a combination of shallow seismic refraction and H/V spectral ratio that yields the Vs-30 and fundamental frequencies correspondingly, are applied for classification. The prepared strong motion database is including events with Mw=2.7. Therefore different lower limit is considered for the used database. The results compared through visual and statistical evaluation of the residuals and most reliable model is selected. Since the lognormal model is considered for the study of attenuation relationships, the residuals have to follow a normal distribution that is evaluated statistically, besides the visual inspecting. The new attenuation relationships, in which the near field saturation is considered for the first time in Iran, are derived for horizontal and vertical PGA. The results are compared with those of worldwide relationships that generally used in Iran and those derived using Iran strong motion data set, partially or completely.