1999年9月21日凌晨發生規芮氏模7.3的集集地震，其觸發車籠埔斷層，且因震源深度極淺（約8~9公里深）、震央靠近台灣中部的人口密集區，遂於斷層沿線造成嚴重災情。車籠埔斷層在形貌上近『倒L』型，『倒L』之長邊為南北走向，北起台中縣石岡，南至南投縣桶頭，長約80公里，地震破裂大多沿著錦水頁岩出露地表，為西部麓山帶與台中盆地之分界；『倒L』之短邊為東西向，從石岡至苗栗縣卓蘭，在石岡附近，沿著大甲溪河床分佈，產生數條地表破裂，至校栗埔改沿著東勢背斜分佈直到卓蘭鎮之內灣，寬約1?2公里，長約12公里，並在石岡附近產生最大錯動量（垂直位移達9.8公尺），此一東西向破裂以往並未有文獻記載，其直接切穿卓蘭層出露地表，錯動行為不同於南北向的主斷層，其形貌與機制極為特殊，需要更進一步的詳細調查。 本研究共採用兩種反射震測法，一為淺層反射震測，為本研究之主力；另一為深部震測法，由國科會委託中油公司進行施測。本研究利用『廣域佈線』之方式，欲將等間距的短剖面經解釋後延伸為連續之長剖面，並將焦點置於區域構造之解釋而非傳統的斷層位置尋找。 1. 車籠埔斷層在大甲溪南岸受TT-boundary阻擋，造成九二一地震之斷層跡東轉並沿著東勢背斜分佈，造成巨大之位移量（垂直位移最大可達12公尺），但在斷層下盤的鄰近地區並未受到影響。 2. TT-boundary（大甲溪-東勢邊界）為本文中所定義之東西橫向阻礙構造，位於大甲溪南岸，西起石岡，往東經過校栗埔後轉為東北-西南向，並沿著東勢背斜分佈。其形貌為一東西向斷坡（南傾20度，位於大甲溪以南）與淺部斷坪（水平?南傾9度，位於大甲溪以北）交接之轉折處，受覆瓦狀構造壟昇影響，造成錦水頁岩往北快速抬昇並接近地表（在大甲溪南岸約800公尺深），當集集地震累積之應力由南往北傳遞至此，遂沿著斷坡爬升，並切穿卓蘭層，造成一序列地表上之東西向破裂。 3. 車籠埔斷層由北往南共有四個地下構造高區，分別為豐原高區（TT-boundary）、車籠埔高區（?子溪至草湖溪）、草屯高區（乾溪至烏溪南岸）與名間高區（平林溪至濁水溪），各高區之頂部與斷層之地表破裂峰值有良好之對應，而GPS同震垂直位移峰值則可對應至斷坡底部。 4. 車籠埔斷層在形貌上與彰化斷層相似，且四地下構造高區可分別對應至大安溪分段線、沙鹿、花壇（包跨大肚溪鑿穿口與花壇）與田中等高區，由此對應關係推測在研究區域內存在方位角為305度之東西向構造，且此東西向構造為控制斷層形貌之主要機制。 Investigating geometry of the Chelungpu fault, Central Taiwan, by the Seismic Reflection Method Chien-Li Lee Abstract The Chelungpu fault, reactivated by the 1999 Chi-Chi earthquake (M=7.6) in central Taiwan, is a bedding thrust slipping along a weak shale layer. The fault trends in the NS direction for 90 km with the eastward average dip angle about 30 degrees. The rupture behaviors of the Chelungpu fault were quite peculiar at its northern end, 50 km north of the epicenter, including: 1) turning 90o to the east (an extra EW branch 12 km); 2) inducing huge slippages (9.8 m vietically); 3) creating high displacement and low acceleration seismic waves. These special rupture behaviors make the Chi-Chi earthquake relatively ‘unique’. More than 300 shallow seismic lines (each 500 m long approximately) and 7 deep seismic lines (one is NS orientation, the others are EW orientation and 25 km long for each) were collected to investigate the Chelungpu fault. The deep reflection profiles provide the background structural distribution and the shallow seismic sections fulfill the gaps to build up the whole structure pattern. A 3D structure was thus established after compiling all seismic images. Three significant findings are proposed: 1. At the northern end, the fault structure has become relatively shallow (less than 1 km). In addition, the fault surface here raises quite fast (25o up-dip) and forms a lateral ramp structure trending in the EW direction, called the TT-boundary (along the Tachia river and the Tongshih anticline). It is believed that this TT-boundary acts as a barrier to interrupt the rupture from the southern epicenter and cause abnormally large displacements at the northern end of the fault; 2. The Chelungpu fault surface (100 km x 20 km) dips 30o-45o to the east as well as 10o-15o to the south. This subsurface is interestingly in an undulation shape. There are four convex at FengYuan, Chelungpu, Tsaotun and Mingjian. These up-rising parts corresponded to abnormal peak of surface rupture. 3. The surface traces between the Changhua fault and the Chelungpu fault are similar and sub-undulated structures correspond to each other. It implies that a deep E-W trending structure controls regional geomorphology. 4. The epicenter of ChiChi earthquake may trap by lateral ramp (dipping 20 degrees to south approximately) and frontal ramp (dipping 50 degrees to east) where is 8~9 km deep at north bank of the Choshuihsi river.
