| Summary: | 博士 === 國立中央大學 === 地球物理研究所 === 93 === ABSTRACT
The 1995 Kobe, Japan, earthquake and the 1999 Chi-Chi, Taiwan, earthquake, both induced several thousand human casualties and significant property losses near their source areas. The 2002 Hualien offshore, Taiwan, earthquake induced strong ground-motion in the Taipei basin, and caused minor damage near its epicenter but significant damage in Taipei, about 110 km from the epicenter. Taiwan is located in the circum-Pacific earthquake zone with a high population density. Earthquake source rupture process and strong ground-motion studies are always important issues in Taiwan to reduce seismic hazards.
The first part of this thesis is “Numerical modeling for earthquake source imaging: Implications for array design in determining the rupture process.” The aim of this study is to develop a numerical method to evaluate the optimum seismic station distribution for imaging the source rupture process of an earthquake. Based on the beam-forming technique, the source rupture distribution of an earthquake can be reconstructed through theoretical travel time correction and waveform stacking. Numerical tests show that this method successfully reconstructs the main displacement distribution from an assumed source rupture plane. In accordance with assumed fault models, seismic waveforms are numerically generated as input data for further source imaging. From synthetic seismograms, we reconstructed the rupture distribution of these assume earthquake sources, and analyzed error systematically. Results of this study indicate that receiver distribution types really affect the successful reconstruction of the slip distribution. Furthermore, parameters such as dip angles and frequency content also play important roles in reconstructing earthquake sources. The proposed method is simple, inverse efficiently, and no initial condition required. Further applications of this method are suggested to image source rupture from near field strong motion observations and to design seismic array to effectively observe seismic rupture properties.
The second part of this thesis is “Simulation of near source two-dimensional wave field and its application to the study of ground motion characteristics of the 1999 Chi-Chi, Taiwan Earthquake.” The characteristics of near source two-dimensional strong ground-motion resulting from complex fault rupture processes and velocity structures have been examined based on two-dimensional wave field modeling. To construct the two-dimensional surface seismic wave field, the synthetic seismogram of each grid space was simulated by theoretical Green's functions. Numerical experiments were constructed by testing different source parameters and velocity structures. The analysis undertaken in this study can be considered as a two-dimensional seismic waveform analysis and offered as a wider view for studying the wave propagation from a large earthquake. Results of this study provide significant information about the temporal and spatial wave field snapshots on the near source area. It is found that the wave fields are strongly affected by the changes of fault geometries, rupture velocities and near fault seismic velocity structures. In this study, the newly developed wave field simulation procedure is applied to analyze the near source ground motion characteristics of the 1999 Chi-Chi, Taiwan earthquake. Summing up the modeling results and comparing with the observed near source wave field of the Chi-Chi earthquake, we find that the Chelungpu fault has lower seismic velocity in the footwall than in the hanging wall, and seismic velocities of the footwall side, at least on its surface, are lower than its apparent rupture velocities.
The third part of this thesis is “Using the frequency-wavenumber spectrum analysis method to analyze the complex effects of strong ground-motion due to the March 31, 2002, Hualien offshore earthquake in the Taipei basin.” The ground motion snapshots in the northern Taiwan area during the MW 7.0 eastern Taiwan offshore earthquake of 31 March 2002 have been reconstructed (Huang et al., 2002). Those snapshots displayed complicated wave propagation and complex direction of ground motion in the Taipei basin. The major shocks during the earthquake of 31 March 2002 were arisen from S-wave later phases and dominate in its radial direction. Those phases could be basin induced surface waves, which converted from body waves through complicated topography of Taipei basin, and made the large shock in the eastern edge and western portion of Taipei basin. In this study, we tried to investigate the characteristics of the converted seismic wave amplitude in different frequency bands with respect to the basin topography and rock site properties. The recorded ground motions from dense seismic network have been analyzed by frequency-wavenumber spectrum analysis method. Results of this study show that: 1. The seismic wave phases have strongly bent through it pass the Taipei basin. 2. The large-amplitude phase (SmS) and the site amplification effect on ground motion were responsible for the maximum peak ground motion associated with the significant damage in the Taipei basin during the Hualien offshore earthquake. To rely on frequency-wavenumber spectrum analyze, results of this study could provide the researchers much useful information to constrain the further three-dimensional numerical simulation for the basin response and velocity structure, and to predict ground motions of the further large earthquakes.
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