Assessment Of Tsunami Amplitudes In The Nearshore Area Of China Marginal Seas Based On MOST Propagation Model

A tsunami is one of the most catastrophic natural disasters in terms of casualties and economic losses. The tsunami forecast model developed by Pacific Marine Environmental Laboratory is based on the Method of Splitting Tsunami (MOST) and simulates the three stages of tsunami:generation, propagation and coastal inundation. The tsunami forecasting system based on this model has been proven to be reliable for the prediction and/or modeling of tsunami impacts in both far- and near field for more than 40 tsunamis since 1996. It has three key components:deep-ocean observations of tsunamis in real time, refine the tsunami source based on deep-ocean observations and tsunami source function in tsunami propagation database, and high-resolution inundation forecast models. This dissertation shows three studies based on this system:detiding wave surface records for extraction of tsunami signals, development of global tsunami propagation database, and assessments of Offshore Tsunami Amplitude along China Coasts.During a tsunami event, deep-ocean observations of wave surface including tidal fluctuations, a tsunami signal, and background noise, it is necessary to extract the tsunami signal to refine tsunami source. In this study, Empirical Mode Decomposition (EMD) is used to solve this problem. The Linear-Operator EMD (LO-EMD) is proposed based on classical EMD method. A type of intrinsic mode function (IMF) that shares much the same original signal in high-order derivative space is defined, one linear operator based on an optimization method is developed to extract these IMFs. This approach shows its outstanding merits in higher frequency resolution and greater robustness for noise disturbance. It is applied to extract tsunami signals from wave surface records. Compared with Harmonic modeling method, this approach uses short data series and achieves certain accuracy.The inversion of tsunami source is based on.deep-ocean observations and tsunami source functions in tsunami propagation database during an event. The current propagation databases contain three separate databases covering the Pacific, Indian, and Atlantic oceans, with 24-36 hours simulation times. In this study, the global tsunami propagation database is developed by MOST model with updated global bathymetry and new boundary conditions. To ensure accuracy, we conduct one dimension and two dimensions wave transformation numerical testing and validate the model using the 2011 Japan tsunami. The modeled wave amplitude and velocity agree well with the analytical solutions. Modeled amplitude time series at tsunameter stations for the 2011 Japan tsunami indicate that the first wave results are nearly identical between the new global database and the old Pacific database. However, the global database can improve the modeled large later waves at both deep-ocean tsunameters and coastal tide stations. In addition, the new boundary conditions introduce less numerical dissipation. Our model results also show linear wave dynamics of tsunami propagation in the deep ocean for both amplitude and velocities.To evaluate the offshore tsunami amplitude along China coasts, the history earthquake records in Global CMT catalog is adopted by MOST propagation model to simulate the corresponding tsunamis. The comparisons of the records and the calculation of five history tsunamis illustrate the effectiveness of this method. The results from MOST model and Boussinesq model drive us to use the bathymetry grids with 1 arc-min when using MOST model and the offshore tsunami amplitude in depth 50 m when accessing the tsunami amplitude along China coasts.Statistical property is analyzed for the offshore tsunami amplitudes in depth 50 m along China coasts and their long-term return value are calculated. The threshold selection method from generalized Pareto distribution validates the rationality of only studying the history earthquake with moment magnitude over 7 degree. The Poisson distribution is used to fit the frequency of history earthquake with moment magnitude over 7 degree, the Poisson-generalized Pareto distribution and the Poisson-lognormal distribution are used to fit the seismic moment magnitude and the offshore tsunami amplitude separately. With the assumption that the return period of March 11,2011 Japan tsunami is 500 years, we calculate the long-term return values of offshore tsunami amplitude along China coasts. If the return period is set as 1000 years, the return values of offshore tsunami amplitudes in Guangdong province northeast seas, Fujian southwest seas, Taiwan southwest seas, Zhejiang northeast seas and seas near Shanghai are above 20 cm; especially when the return period is 2500 years, the offshore tsunami amplitudes in Guangdong northeast seas, Fujian southwest seas and seas near Taiwan Kaohsiung are over 50 cm. The Gumbel Copula function is used to construct the joint distribution of seismic moment magnitude and offshore tsunami amplitude along China coasts. If the joint return period is 1000 years, the area where the offshore tsunami amplitudes are over 20 cm is larger than the one dimension distribution analysis case, but still around the provinces shown in one dimension case.