Dynamic simulation of ground motions from scenario earthquakes on the North Tehran Fault

The North Tehran Fault (NTF) represents an important source of seismic hazard to the city of Tehran, Iran, as it is known to be the causative fault of large historical earthquakes. In this study, the characteristics of long-period near-field ground-motions in the city of Tehran for M7.2 scenario earthquakes on the eastern segment of the NTF have been investigated using dynamic rupture simulations. Taking into account the NTF geometrical features, wave propagation properties and characteristics of rupture dynamics, realistic low frequency (0-2 Hz) ground motions are calculated using 2-D spectral element simulations. Various self-similar initial stress distributions provide a wide range of rupture propagation complexities, rise-times and final slip scenarios, which are statistically consistent with empirical attenuation relationships for the region. Near-fault wavefield features, such as long-period pulses, are obtained from dynamic simulations and their effects on the ground-motions are discussed. The model takes into account the Tehran basin structure, which is composed of deep soft alluviums, by including a 2-D shear wave velocity model which comes from the most recent studies of the region. Our ground-motion simulations reveal that the low-velocity sediments of the Tehran basin have major effects on the amplitude, frequency content and duration of the simulated time-histories. The simulated ground-motions from these scenarios are generally in agreement with one of the Next Generation Attenuation models for shallow crustal earthquakes in active tectonic regions. The mean 2-s horizontal spectral acceleration in the northern parts of the city is about 0.46 g, while near the midtown this value is up to similar to 0.26 g above the deepest parts of the basin. The basin effects are higher on deeper parts and higher frequencies.