數值化地形變成像及斷層運動行為分析

Abstract

地震的發生常造成地表破裂或變形，它的發生在科學上已被理解是地殼應力在斷層面上累積再釋放的結果，然而發震斷層在地底下的幾何型態則與地表破裂與變形息息相關，無論是在科學還是災防考量上，了解斷層幾何形貌及斷層運動機制都是重要課題，為了深入探討這個關鍵議題，必須解析同震、震後或是間震期間斷層運動的完整發展過程，因此，本研究試選了幾種不同的方法在不同的地震發生地來進行比較與分析。針對2008汶川地震與1999集集地震，本研究採用光學成像的衛星和航空照片，利用像素比對的方法，得到兩個大地震事件中，高精度的同震地表水平位移，若能利用其他方法取得垂直位移，藉由兩者位移的合成，將可進一步重建詳細的發震構造幾何，而所得到的斷層幾何模型配合地表位移資訊，將可進行反演模擬其破裂行為的特性；在汶川地震之前的研究中，靠近地表破裂的地表位移並不清楚，利用衛星影像的比對，可獲得連續地表位移的大小與方向，則進一步協助了解地震發生當時地表破裂的發展過程；在集集地震的例子中，藉由成對的航空照片進行匹配，獲得的地表位移可達每百公尺一個格點資料的精度，也結合高程差來還原高精度淺部地殼下的斷層幾何型態。此外，本研究也在西索羅門群島隱沒帶斷層上佈建連續GPS觀測網，不僅可以監測同震的資訊，也可對間震期的地殼變形進行分析，因此可以進一步在時間軸上分析發震斷層面上的滑動耦合情形，據此也可以再討論大地震發生的可能性。總之，不同的測量方法各有其優點和缺點，會因不同地區與不同的科學目的，選擇不同或數種方式來獲得地表資訊，期許之後能結合各方法所得到的數據，做一綜合研判來確保結果的完整性與合理性。

Earthquakes are the result of releasing accumulation stress on the fault plane underground and large ones often produce surface ruptures and deformations, which are closely related to the fault geometry down beneath. For both the purpose of scientific approach and hazard mitigation, to reconstruct the fault geometry and diagnose related rupture behaviors are topics of great importance nowadays. In order to proceed towards the insights in this research orientation, this study works on the fault geometry and comprehensive process of the fault kinematics in the durations of coseismic, postseismic, and interseismic. Several different methods were applied on real seismic cases. The optical images of satellite and aerial photos are applied by the technique of pixel comparison to generate high resolution mapping of coseismic horizontal displacements. If the vertical displacements can be also generated by other independent methods, the detailed seismogenic fault geometry can be reconstructed by synthesizing above horizontal and vertical component. For the Wenchuan earthquake, the derived amplitude and direction of the surface displacement are of critical assistance to help understand the behavior and propagation of coseismic fault rupturing. For the case of the Chi-Chi earthquake, because the source images are paired aerial photos, the mapping of horizontal displacement can reach a resolution of 100 meters in grid. Also resulted is equivalent high resolution 3D fault geometrical model. The last part of this dissertation is to set up a continuous GPS network across the subduction zone to monitor the ground coseismic to interseismic deformation in western Solomon Islands. Based on the observation data, the coupling ratios on the interface fault within the subduction zone can be estimated and further used to discuss the occurrence potential of large earthquakes. As a whole, different methods have their own prevailing aspects and weaknesses. The best way to obtain reliable results is to select the most suitable one under the consideration of regional characteristics and various scientific purposes. Nonetheless, to synthesize results from different projects using different techniques may help ensure the comprehension and rationality of the scientific findings.