九二一集集地震三維震源過程與震波傳遞分析; 3D Rupture Process and Wave Propagation Analysis of the 1999 Chi-Chi, Taiwan, Earthquake

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Title:	九二一集集地震三維震源過程與震波傳遞分析;3D Rupture Process and Wave Propagation Analysis of the 1999 Chi-Chi, Taiwan, Earthquake
Authors:	李憲忠;Shiann-Jong Lee
Contributors:	地球物理研究所
Keywords:	三維震源過程;九二一地震;三維震波傳遞;Chi-Chi Earthquake;Source rupture process;3D wave propagation
Date:	2003-06-03
Issue Date:	2009-09-22 09:53:41 (UTC+8)
Publisher:	國立中央大學圖書館
Abstract:	震源過程分析與強地動模擬在地震物理及危害分析上已是一個非常重要的議題，而高精度的三維震源過程逆推與三維震波傳遞的模擬在這個議題中更是扮演著舉足輕重的角色。在台灣地區中，近年來最受矚目的是1999年9月21日ML=7.3之集集大地震，雖然這個地震為台灣帶來莫大的生命與財產上的損失，卻也提供了地震學者一個絕佳的機會來重新檢視大規模地震的震源與波傳行為。本研究結合平行化三維波傳有限差分法與並列線性逆推技術來分析九二一地震所記錄到之龐大而複雜資訊，接近完整地還原了地震發生時的震源與強地動過程，有助於釐清各區域的強震生成過程。其成果使九二一地震於保存密集而高品質的地震記錄資料外，亦成為世界上少數成功結合高精度三維震源與路徑效應來詳細分析大尺度強地動過程的災害性地震。透過並列線性逆推技術所求得之高精度震源逆推結果指出，921集集地震的震源總錯動持續時間接近60秒，其破裂過程不論在靜態或動態上均極為複雜；靜態破裂分析指出，主要錯動發生在深度10公里以上的斷層淺部，最大錯動量15.8公尺位在斷層北端轉折區，並向下延伸至15公里深處，此極大錯動區屬於同一發生機制而非獨立事件；動態破裂分析中更可明顯看出，斷層面上南北兩端有著截然不同的破裂行為，南端由滑移速率小但次數較多之重複滑移所構成，錯動持續時間長達30秒，而北端極大破裂區之破裂行為主要由滑移速率極高、持續時間僅10秒之單一錯動所支配。與過去的921地震震源研究做比較，本研究於靜態破裂分佈的解析能力有很大的提升，即使就細部的動態破裂過程來看，此次的分析確實發現許多過去所無法完整呈現的現象。這些高精度的靜態、動態破裂過程，對於後續震源物理與強地動預估等相關研究將可提供更為詳盡的訊息。結合高解析度破裂過程與平行化三維波傳技術，本研究成功重建了921地震所造成的強烈震動過程。結果指出，車籠埔斷層所產生之地震波動於不同的區域有截然不同的特性與生成過程。複雜的震源破裂行為、斷層面幾何模式與傳播路徑上的速度側向變化等均造成震波隨震央距離有不同的衰減與震動特性。由於車籠埔斷層的低角度逆衝特性，強震動區域明顯集中在斷層上磐，特別是斷層北端之豐原-石岡附近，其下方Aspersity所釋放出的強烈振動能量為震源不均質性最直接而明顯的影響。斷層北方新竹-苗栗一帶受到震源過程中破裂朝北傳播的影響，沿破裂傳播方向上有明顯的Directivity效應，造成強烈的地表振動。而在斷層西部的台中-彰化一帶，雖然座落於下磐區，卻由於三維速度模型中淺層低速區的作用，普遍出現表面波震幅放大現象。至於嘉義以南的區域，其位處在震源破裂方向的相反處，震波建設性累積的效應無法產生，故震動相對較弱。震波於中央山脈中，受到車籠埔斷層面逆衝滑移特性的影響能量快速衰減，但向東進入海岸山脈後，由於近地表速度梯度的急遽變化而產生些微放大。整體而言，集集地震發生後於各地區之強地動的發生行為與過程主要仍是受到強大的震源效應所支配。綜合以上的研究結果指出，結合強地動觀測資料、震源過程研究、精細的三維速度構造與數值模擬技術將是於未來實現強地動預估的重要關鍵。 A complex source process inversion technique including three-dimensional fault geometry, three-dimensional Green's functions and parallel non-negative least square inversion are used to explore the high resolution spatio-temporal slip distribution of the 1999 Chi-Chi, Taiwan, Earthquake. The 3D fault geometry was inferred from the aftershocks distributions and high-resolution reflection experiment studies. It shows dip angle of the fault becomes shallower from south to north and near flat at the deeper portion of the fault. The 3D Green’s functions were calculated from a 3D finite-difference code using 3D velocity structure derived from tomographic studies for grid size of 0.5km and time interval 0.05sec. The 3D Green’s functions show significant azimuthally variation of the waveform suggest the necessity of the 3D Green’s function calculations in Taiwan. On account of the 3D fault geometry and 3D Green’s functions, we use parallel linear inversion technique to invert the spatial/temporal slip distribution of the 1999 Chi-Chi earthquake using the dense strong motion waveforms considering huge amount of multiple time windows. Finally, the 3D source parameters derived from the inversion result was used to forward modeling the full-wave 3D wave propagation of the 921 earthquake. Our study here emphasizes the necessary of 3D fault geometry and 3D Green's function that correctly account for both the complicated source process and wave propagation effect of the large damaged earthquake. The major achievements of this research are stated below : A.High resolution 3D source process inversion Final result is drive from 48 time-windows inversion. The rupture process shows very complex spatio-temporal slip behavior. Major slip area is located at the shallow part of the fault (about 10 km depth). The largest slip, 15.8m, is at about 30 km north of the epicenter which is near the bending of the fault and extend to about 15km depth. The rupture time history at southern and northern parts is entirely different. In southern part, the slip behavior is composed by repeatedly and small slip rate; while the northern part is dominated by single, duration about 10 sec, large slip rate event. Comparing with previous results, the resolution of the static slip distributions is more refined in this study. The same with the dynamic rupture process, there is some phenomena that can only be revealed detail in this study. These high resolution source process analyses can help to promote further study in earthquake physics and strong ground motion prediction. B.Forward strong ground motion simulation of the 921 Chi-Chi earthquake Joining the high resolution source process model and parallel 3D wave propagation technique, we have successfully reproduced the strong ground shaking during the Chi-Chi earthquake. Comparisons between the simulation results and observed waveforms from a dense strong motion waveforms clearly demonstrate that the variation of the velocity structure and the complex fault slip process greatly affect the damage during the Chi-Chi earthquake. The source directivity effect produced large amplitudes along the direction of the rupture propagation. Low velocity under shallow part of Coastal Plain generated significant amplified ground motion. Under Central Range, the seismic wave was relatively weak owing to the energy radiation mechanism of the Chelungpu fault. The ground motions were then amplified further by the high velocity gradient under Coastal Range. Finally, the strong ground motion characters at different regions were mainly dominated by intense source effects. The results of this research point out that a close link with strong motion observations, source studies, detailed knowledge of velocity structure, and numerical simulation technology is necessary to make the prediction of strong ground motion feasible in the further.
Appears in Collections:	[Graduate Institute of Geophysics] Electronic Thesis & Dissertation

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