The large number of strong motion data from the national network of strong motion Iran (NSMNI) motivated this study to develop relations for prediction of response spectra ordinates for periods in the range of 0.1 to 3.0 seconds of horizontal acceleration components of the strong-motion records. We determined attenuation relations of spectral responses to calculate 5% critical damping as a function of spectral periods, and in terms of earthquake magnitude, source to station distance, and recording site conditions. The two horizontal acceleration components of a total of 883 strong-motion records compiled to develop a predictive ground motion model for Iran and its three main seismotectonic regions, Alborz, East and Zagros, and also for the central south of Iran. The data set were recorded from 79 earthquakes from 1987 to 2007, and with moment magnitudes of 5 and larger. A hinged-trilinear form of attenuation model was used for the regression analysis of data set. To estimate the coefficient of attenuation relations we employed the Monte Carlo technique. The results show that while some coefficients are strongly dependent on frequency, some are fairly independent on frequency.

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