Numerical simulation of near-field pulse-like ground motion for the Shuantung fault in Taiwan region

Based on the geological and geomorphological characteristics of western Taiwan and the source parameters of the 1999 M_W7.6 Jiji （Chi-Chi） earthquake, we have established a 3D velocity structure model and two types of source models. Based on the accumulation of dislocation in the crust and the propagation characteristics of stress and strain after the rock fracture, 3D finite difference method was used to simulate the near-field pulse-like ground motion that would occur in the Shuantung fault activity. The results show that the peak velocity of the horizontal component perpendicular to the fault strike of the strike-slip fault is large, and so is the peak velocity of the vertical component of the reverse fault. The double-sided velocity pulses generated by the directivity effect are mainly concentrated in the direction perpendicular to the fault sliding component, while the single-sided velocity pulses generated by the fling-step effect are mainly concentrated in the direction parallel to the fault sliding component. Because of the mutual control of the directivity effect and the hanging wall effect, the near-field pulse-like ground motions exhibit an asymmetrical zonal distribution, and the velocity pulses mostly distributed within 15 km from the strike-slip fault trace and 10 km from the reverse fault trace. The velocity response spectrum gradually increases along the rupture direction within the coverage of the fault plane, and the velocity pulse may cause severe shear damage to large buildings. Influenced by the characteristics of the asperities, the seismic wave field shows that Nantou, Taichung, and Miaoli are in risk region of strong ground motion.