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Abstract:
Accurate assessments of tsunami risk are a prerequisite of effective disaster prevention and mitigation. Traditionally, seafloor dislocations calculated in the elastic semi-infinite space are given as initial tsunami distribution, and then tsunami propagation is studied numerically. Since the natural fault rupture is incompletely instantaneous, the initial tsunami heights will be less than seafloor dislocations, that is, initial tsunami attenuation (ITA). At present, the quantitative relationship between ITA and the relevant tsunami wave heights remains unclear. This quantitative relationship is investigated in this study through constructing a series of high-precision hydrodynamic numerical models: a case study of the South China Sea (SCS). Two significant controlling factors are involved in our numerical models: the magnitude of ITA, and the segmentation of the Manila fault zone (MFZ) which is assumed to lead the tsunami in the SCS. It is no surprise that ITA can reduce the tsunami height in the SCS in our models. Numerical results further reveal that the ratio of tsunami height reduction is directly proportional to the magnitudeof ITA with a negligible error (2.5%). That means if we know the magnitude of ITA, we will immediately obtain tsunami wave heights without performing numerical modeling. With a conservative initial tsunami attenuation (10%), the simulation results show that the southeast coast of China, the east coast of Vietnam and Palawan Island are tsunami hazard zones. In addition, the model shows that the magnitude of wave height due to Coriolis force is less than 5 cm and the distribution pattern is as expected, which further confirms the reliability of the numerical model and shows that the Coriolis force can be ignored in the actual South China Sea tsunami simulation to improve the computational efficiency.The combination of the previous sedimentary evidence and the current numerical experiment suggests that a 1000-year-ago damaging tsunami occurred in the SCS, most likely resulted from the southern segment of the MFZ, and was geologically recorded in three isolated places: Nan'ao Island, Dongdao Island and the eastern of Vietnam but Yongshu Reef. These places may be good choices to perform detailed sedimentary studies in order to constrain the tsunami period (or earthquake period).
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