The experience of past earthquakes has shown repeatedly that the characteristics of ground motions and resulting damages are highly dependent on the local condition of the site. It is clearly known that the underlying layers may amplify the earthquake excitation to some extent. This phenomenon is called the site effects. The site amplification depends on several factors such as stiffness, thickness, and density of the subsurface layer. In the present article, the 1D yet acceptable square‐root‐impedance method is used to estimate the frequency‐dependent amplification function for various site classes in Iran. To achieve this goal, a stochastic method is applied to generate several thousand velocity profiles down to 8000 m. These profiles are randomly generated and constrained based on the features of 458 available shallow velocity profiles and crustal structure models across Iran.
Then, the resulted amplification functions are multiplied by near‐surface attenuation function to obtain the site response in the frequency domain. The comparison of the mean spectra of different site classes demonstrates that although the softer classes (the ones with lower VS30) have higher amplifications at lower frequencies, the trend is completely reversed at higher ones.
Finally, the generated profiles are employed to derive models for Z1.0 (the depth at which the shear‐wave velocity is equal to 1 km/s). This depth is very important, because it is included in the new generation of ground‐motion prediction equations.