Near-fault strong motion complexity of the 2003 Bam earthquake (Iran) and low-frequency ground motion simulation

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The 2003 December 26 Bam earthquake was an intermediate size strike-slip event that occurred beneath the Bam city, causing more than 26000 fatalities. The record of the only near-fault station (Bam station) shows a clear long-period pulse in almost fault normal direction suggesting a near-fault forward directivity effect contributing to the heavy damage observed in Bam city. In this study, the long-period (0.1–1.5Hz) strong ground motions were simulated to answer some ambiguities in source parameters for fully description of the observed motion in different components at Bam station. To this end, the Hisada's kinematic model accounting for both heterogeneous source characteristics and underground geology is applied to simulate the long-period ground motion. Different source scenarios suggested by previous studies are examined and the optimum scenario that could provide the best fitting between the simulated and observed motions for different components was introduced. The slip model and asperities as well as velocity structure parameters are optimized in order to have the best possible fit between the simulated and observed velocity time histories and spectra. The good agreements between the simulated and observed low-frequency motions indicate a successful identification of source and slip parameters of the Bam earthquake and provide a good test for the simulation technique. Furthermore, the uniqueness of the identified source parameters and the slip model is further checked against the observed near- and far-field recordings of the Bam earthquake. A good agreement between the simulated and the observed waveforms provides a base for model verification and results confirmation.