台灣嘉南地區的發震構造分析及其前緣斷層的長期滑移速率研究

Abstract

1999年921地震後，探討台灣造山帶前緣的發震構造為一刻不容緩的課題。為了瞭解台灣西南部的斷層活動，我們整合區域地質、地球物理及大地構造資料，以建立地下斷層的模式，並配合大地測量及地震分布型態，提出西南部造山帶前緣的發震構造。 地處碰撞造山帶的台灣，西南部造山帶前緣包含西部麓山帶及海岸平原區，其中西部麓山帶的地形崎嶇，岩層多褶曲斷裂，存在一系列以北東向的逆斷層出露於地表。由逆衝斷塊的分布型態，此一褶皺逆衝斷層帶，大致以獭頭斷層為界區分為嘉義區塊及台南區塊。海岸平原區的地形平坦，岩層大致平整，然而由地下地質及構造地形的分析顯示，造山帶前緣以一向西逆衝的盲斷層潛伏於海岸平原區之下，為造山帶及前陸盆地的分界。此外，海岸平原區之下亦存在一系列早期發育且近乎東西向的正斷層，並以義竹斷層為界將中生代的基盤區分為北淺南深的基盤形貌，可分別對應至嘉義區塊及台南區塊。此一結果顯示中生代基盤的形貌著實影響褶皺逆衝斷層帶的發育，然而此基盤並未加積至造山帶前緣內。由平衡剖面的分析，無論嘉義區塊及台南區塊均可由一楔形的造山帶前緣來詮釋，各逆衝斷塊座落於一向東微傾的滑脫面上，可視為嘉南地區主要的發震構造。 分析晚第四紀岩層的垂直變形及近期大地測量的位移型態，嘉南地區造山帶前緣的斷層具有較高的活動潛能。在嘉義區塊為嘉義盲斷層及九芎坑斷層，台南區塊為新營盲斷層及六甲斷層。透過平原區鑽井資料的層序地層分析及定年資料顯示，新營盲斷層開始活動的年代為39,245±215 yr BP，其長期的滑移速率為2.4±0.6 mm/yr，而六甲斷層的長期的滑移速率為14.3±0.3 mm/yr。整合發震構造及長期滑移速率的計算，可推估新營盲斷層發生大地震的規模約為7.26 (Mw)，再現週期約為742年；六甲斷層大地震的規模約為7.01 (Mw)，再現週期約為100年。

Taiwan, the young orogen that resulted from the collision of the Luzon arc and the Eurasian continental margin, exhibits strong seismic activity. Since the 1999 Chi-Chi earthquake, the government has conducted many projects on active fault study in Taiwan, documenting more than fifty active faults around the island. Recently, a neotectonic map of Taiwan was developed, showing major active deformation belts around the island and 11 domains that are distinguished by geomorphic markers, geodetic and seismologic data. A case study of the 1999 Chi-Chi earthquake shows that the frontal thrust belt of central Taiwan exhibits as a imbricate thrust system that is locked and rooted into an aseismic décollement creeping at about 27~42 mm/yr. In addition, based on foreland basin retrieval and river terraces investigation, the sum of the frontal active Changhua blind thrust and the Chelungpu thrust was found to have about ~32 mm/yr of long-term shortening rates. These results indicated that there is a highly crustal strain accommodation at the frontal thrust belt where disastrous earthquakes would occur in near future. Similar to that of the Central Taiwan, the Chiayi-Tainan area also has a locking frontal thrust belt, but the traces of active faults in this area are mainly less than 20 km, such as the Chiunchungkeng fault, the Muchiliou fault and the Liouchia fault. Indeed, the Mesozoic basement (the so-called Peikeng High) is underneath the Chiayi area at a shllow depth of nearly ~ 2 km, and suddenly drops deeper to ~ 6 km southward. A pre-existing normal fault, the Yichu Fault, a major boundary trends ENE and serves as that accounts for the separation of the basement. Given these observations, several structural models have been proposed for the frontal thrust belt in the Chiayi-Tainan area, such as a shallow décollement of a thin-skinned collision model, a deep décollement of a thick-skinned collision model, and an inversion of pre-existing normal faults model. As the subsurface geometry of the faults plays a key role in future earthquake hazard assessment, the first chapter of this thesis uses geological and geophysical data from both the Western Foothills and the Coastal Plain to reconstruct the fault geometry underneath the Chiayi-Tainan area. Chapter two of this thesis focuses on the morphotectonic division in the Chiayi-Tainan area by drainage pattern analysis and Holocene vertical movement rates measurement. Analysis of borehole data and drainage patterns of the southwestern part of the Coastal Plain indicate that is the Taiwan orogenic belt. This is interpreted as being caused by a NNE-trending blind thrust beneath the Coastal Plain, and it represents the deformation front of the Taiwan fold-and-thrust belt. The deformation front is thus located between the plain area and tilted tableland area of western Taiwan, westwards of where it had previously been defined. The frontal thrust is proposed to be segmented by several E-W trending strike-slip faults. For example, the Kukeng fault and the Hsinhua fault bound the Chaiyi block to the north and the south respectively, delineating a segment of the fold-and-thrust belt in southwestern Taiwan. Chapter three of this thesis presents the evaluation of slip rates of two frontal thrusts, namely the Hsinyin blind thrust underneath the Coastal Plain and the Liouchia fault along the mountain front. The calculation of the slip rates is based on the borehole data and subsurface geometry of the faults that reconstructed in chapter one. These results indicate that the two faults have about ~18 mm/yr long-term shortening rates regarding the arc-continental margin collision. As the highly crustal stain has been accommodated in the frontal orogenic belt, the final part of this thesis contains present the implications for earthquake hazards in the study area. The following is the list of chapters Chapter 1 A basement impinged frontal orogenic belt of southwestern Taiwan and its implication for the subsurface seismogenic structure Chapter 2 Active deformation front delineated by drainage pattern analysis and vertical movement rates, southwestern Coastal Plain of Taiwan Chapter 3 Timing and slip rates of the frontal thrusts from late Pleistocene-Holocene borehole data analysis, southwestern Taiwan