| 摘要: | 論文提要 1995年阪神地震，造成日本阪神地區人員嚴重傷亡，城市的重建與受災人民的安置經歷數年未盡，龐大的經濟財產和社會成本損失無以估數。1999年921地震，強大的震波能量造成台灣地區許多樓房倒塌傾毀，除了有價物質與社會成本損失外，受傷喪生的生命損失更無法以金錢來衡量。台灣與日本同處環太平洋地震帶，對於強震防災更應以此經驗為鑑。本論文的第一部份研究為「震源成像模擬：探討觀測震源破裂過程之地震儀陣列最佳化設計」。此部份章節提出一套簡單實驗，利用波束聚集法（Beam-forming Method）反求震源的破裂過程，找出觀測震源破裂的最適化測站分佈，對地震儀陣列的規劃提供重要訊息以應用於潛在斷層區作不同的測站分佈模擬，並可加以參考現有地質資料、斷層構造、與地震活動（seismicity）分佈，求出最適測站陣列分佈，以期利用有限測站獲得最佳的破裂過程解析，提供地震網設計規劃之參考。本論文的第二部份研究為「地表強地動時空分佈模擬：探討1999年9月21日集集地震所引致之地表強地動時空分佈特性」。此部份研究是藉由計算近斷層地區強地動的時空分佈特性，去調查斷層的傾角、地下介質速度、與斷層破裂速度是如何的影響地表強地動能量的分佈，並嘗試去歸結在這些因素影響下地表強地動時空分佈所顯現出來的特徵，以釐清斷層破裂過程與地下速度構造所造成之地表近場二維強地動分佈有何顯著特性。本論文第三部份研究為「利用頻率–波數頻譜分析法分析2002年3月31日台灣東部外海之331地震在台北盆地所引致之強地動複雜效應」。此部份研究利用氣象局強震網於台北盆地所收集到之331地震資料進行分析，藉以了解穿越台北盆地的震波在不同頻段中能量分佈的特性以及震波在入射台北盆地後如何受到盆地的影響，希望釐清台北盆地地形與當地場址岩性對地動能量的匯聚有何影響，並期望能更進一步在數值模擬研究中提供有關台北盆地響應（response）與地下速度構造的資料，進而可以對未來可能發生的大地震進行重要都會區強地動預估，以期達到防災之研究意義。 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. |
