Attenuation relationship for inelastic earthquake spectra in Iran

One of the most important components of a seismic risk analysis procedure is a suitable earthquake attenuation relationship of the area under study. Attenuation relationships express the ground motion parameters and spectral ordinates as a function of the effective parameters. The purpose of this study is to present attenuation relationships describing constant-ductility inelastic spectral ordinates and structural behaviour factors in the Iranian plateau. The plateau is divided into two seismic zones, Alborz - central Iran and Zagros, each of which is subdivided according to the ground type. A total of 954 horizontal ground motions recorded in 490 earthquakes were used. These events had surface wave magnitudes ranging from 4.0 to 7.7 with hypocentre distances greater than 10 km and often less than 200 km. Inelastic spectra for ductility ranging from 2 to 8, within the range of 0.05-4 s, are calculated. A gene expression programming algorithm instead of the conventional constant regression model is used to obtain the attenuation relationship for the inelastic spectra. It is proved that the proposed algorithm can develop the model precisely as a continuous function. The results show a consistency with an acceptable proportionality coefficient among the observed and anticipated results.