利用航空照片之次像素關聯探討同震地表位移：以1999集集地震(Mw 7.6)草屯地區為例

Abstract

當地震發生的地點靠近工商發達、人口稠密處，隨著大地震所伴生之地表破裂，極易造成房屋倒塌或其他破壞，因而產生許多不可預期之災害。為了估計將來地震可能造成之災害，瞭解如何造成地表破裂是非常重要的課題。其中斷層在地底下的幾何型態與破裂機制息息相關。為探討此機制及斷層幾何，間震或是同震期間，詳細且連續之地表滑移量分布是不可或缺的資料。 台灣在西元1999年9月21日發生了規模7.6的集集大地震，地表破裂處造成許多災害。在草屯地區，地表破裂大多沿著車籠埔斷層，除此之外，在其東側兩公里處，尚有一伴生構造（隘寮斷層）造成地表變形，所以應有同震地表位移的改變。前人藉由次像素關聯（Sub-pixel correlation）比對震前與震後SPOT衛星影像並統計處理，而得到同震地表位移，並以此探討大範圍的地表變形及車籠埔斷層的地下幾何型態。但因SPOT衛星影像解析度不高，無法精細地標示出地表破裂的位置，而且於隘寮斷層上也無明顯水平位移改變。 本研究使用更高解析度之航空照片，利用影像正射（Orthorectification）去除地表起伏所造成之變形，也採用次像素關聯將正射後之影像相互比對並配合統計處理，由此獲取更精細之同震地表位移。此結果可清楚地標示出車籠埔斷層造成地表破裂的位置，並且在隘寮斷層上也可觀察出明顯的水平位移改變。由東西向剖面可知，跨車籠埔斷層位移改變約為4.5∼5.5公尺向西，3∼4公尺向北，北段水平位移值大於南段；跨隘寮斷層位移改變僅1∼2公尺向西，南段的位移在較短距離上呈現明顯的改變，且於南段中間出現最大的位移改變量。由此可知，車籠埔斷層面於草屯地區，北段傾角大於南段，與水準測線資料吻合；且地下斷層面在南段中間區域有明顯的轉折。 本研究將次像素關聯應用於空間解析度較高的航空照片，提供衛星影像精度無法達到的結果，除探討斷層破裂行為與斷層地下幾何之間的關係外，並期許若在較小地震發生的區域，亦能使用此法觀測出地表的移動。

When an earthquake occurs close to an urban area, it is easy to cause the collapse of buildings and other damages following by the surface rupture. It also results in unpredictable disasters. In order to estimate the amount of damages following by an earthquake, it is important to understand how an earthquake causes the surface rupture. Furthermore, the surface rupture is closely related to the underground fault geometry and subsurface structure. Thus it is necessary to obtain detail and continuous inter-seismic or co-seismic ground slips to study surface rupture fault and fault geometry. On Sep. 21, 1999, Mw 7.6 Chi-Chi earthquake occurred in Taiwan. It causes many damages at the area of surface rupture. At the Tsaotun area, the surface rupture always occured along the Chelungpu fault (CLPF). In addition, about 2km from the east of CLPF, there was an Ailiao fault (ALF) which also created some surface deformation, thus there should be changes in co-seismic ground displacement. In previous study, a more complete measurement of the co-seismic ground displacement was retrieved from sub-pixel correlation of SPOT satellite images. The displacement distribution was used to study the surface deformation and underground geometry of the CLPF structure. However, it cannot accurately locate the position of surface rupture of CLPF and there were no obvious changes in displacement on ALF due to the low resolution of SPOT images. In this study, the high-resolution aerial photographs are used for analysis. This study also uses orthorectification to eliminate the distortion from the topography and sub-pixel correlation to compare and analyze orthor-images, and then obtain more accurate co-semic surface displacement. The result clearly shows the location of surface rupture of CLPF and the obvious change in horizontal displacement on ALF. By east-west profiles, the changes in horizontal displacement were 4.5~5.5 meters toward west and 3~4 meters toward north cross the CLPF, and the changes of the northern segment was greater than the southern one. The changes in horizontal displacement were only 1~2 meters toward west cross the ALF, the changes zone of the southern segment was shorter than the northern one, and the greatest change was appeared in the middle of the southern segment. Therefore, the dip of northern segment was bigger than the southern one on the CLPF plan at Tsaotun, and closed to the result from level elevation changes. Further, the angle of subsurface plane should be an obvious change in the middle of the southern segment. This study applies sub-pixel correlation to the higher resolution aerial images and provides a more accurate result than the SPOT images can. In addition to the discussion of the relationship between the fault and its underground geometric structure, this method may be able to produce the surface displacement even in the area with smaller earthquakes.