- 學位論文
台北都會區山腳活斷層之地震地質研究
Earthquake geology of the active Shanchiao Fault in the Taipei Metropolis, Taiwan
摘要
擁有近千萬居民的台北都會區面臨多樣的地震災害潛在來源，包括臺灣島周圍造山與隱沒系統相關震源，以及都會區內的活斷層「山腳斷層」。山腳斷層為一東傾之正斷層，斷層線位於台北盆地西緣，其活動主控了台北盆地的生成以及盆地內四十萬年來七百餘公尺厚河相為主沉積物的堆積。潛在的地震可能造成台北都會區大規模的災害，特別是沿著地表斷層線附近地帶、斷層上盤位移量較大地區、以及震央附近地區。為了增進對此活動斷層基本性質的了解，本研究分析了台北盆地地區的水準資料、地形資料與鑽井資料，並更清楚解析斷層實際所在位置、斷層的活動情形、斷層帶構造與斷層幾何。近三十年來(1975-2003)的水準資料顯示，台北盆地的地表垂直變形主要受控於受壓含水層孔隙水壓變化造成的含水層與阻水層變形；考慮自現地實驗觀察所估計的沉積物壓密速率，以及含水層孔隙水壓回覆造成的彈性回彈，盆地西緣五股至新莊地區有較高的下陷速率，指示山腳斷層目前正在潛移。台北盆地西緣山腳斷層帶的數值高程地形模型分析以及現地實測結果可以追蹤描繪出一系列右階雁行排列的斷層相關地形崖，與斷層的左移性質有關。為了探討較長時間尺度斷層的活動情形，首先選取了斷層中段五股地區岩性紀錄與定年資料相當完備三口鑽井的沖積物進行沉積相與年代的整理分析，對比全球海水面變化曲線，以構造剖面回復方式重建了山腳斷層帶自末次冰期以來的生長斷層演育。由生長斷層的同構造沉積物堆疊架構顯示山腳斷層自末次冰期以來持續活動，斷層帶是由一高角度的主斷層以及西側一較低角度的分支斷層所組成，因斷層含有左移的分量而呈現負花狀構造。兩萬三千年來主斷層的平均垂直向構造滑移速率約為2 mm/yr而分支斷層約為1 mm/yr，而九千至八千四百年前的六百年區間內我們紀錄到特別高量的構造沉陷(主斷層7.4米，分支斷層3.3米)，與前人提出該時段內曾發生古地震的觀察相吻合。在山腳斷層帶中北段的蘆洲剖面亦觀察到了相似的負花狀構造並且可判釋出多條分支斷層的存在，斷層上盤末次冰期以來之構造沉陷速率約為3 mm/yr與五股剖面估算所得相當接近，顯示如此之負花狀生長斷層是山腳斷層帶的共同特徵。將地表地形與地下構造對比顯示地形崖多對比至較西側/外側之分支斷層，主斷層的斷層跡則已被快速的侵蝕與堆積作用完全抹除，因此山腳斷層在地表上的分布並非一單獨的線形，而是一個可達數百公尺寬的斷層帶。在生長斷層分析中，末次冰期末期因大漢溪襲奪事件而在台北盆地形成的景美沖積扇礫石層因其形成迅速、分布廣泛、頂面形貌較規則平整、井下辨認容易，成為同構造生長沉積物中紀錄與估計山腳斷層長期、綜合同震與間震期完整地震循環的垂直位移相當可靠的指準層。整合盆地內超過五百口鑽井資料可見此指準層-景美礫石層頂部-已被山腳斷層明顯強烈變形，並呈現roll-over的單斜褶皺形態；末次冰期以來斷層最大位移位於斷層中段的蘆洲至五股地區，並迅速向南減少，向西也快速減少但至盆地中心後減少速度變緩。斷層造成指準層位移的量值與分布為斷層的幾何形貌所控制，本研究選取垂直斷層線的五股-三重-台北區域的景美礫石層頂部深度變化進行模擬，以簡單的彈性半空間邊界元素數值模擬法嘗試解析山腳斷層在上部地殼的幾何形貌；模擬結果顯示山腳斷層在淺部傾角約75至85度接近垂直，然而在三至五公里深處急遽轉折至近水平15至5度，呈現強烈的匙狀幾何，指示山腳斷層在地下三至五公里深處與造山時期的逆衝斷層結合並構造反轉重新滑動。考量山腳斷層為一可能發震的斷層，而全球至今尚未紀錄到低角度正斷層所產生的中大型地震，低角度正斷層的發震機制亦缺乏適當的力學解釋，同時進入台灣造山帶的中國大陸邊緣在造山運動之前發育有許多地塹相關的正斷層為先存弱面，故此研究在上述幾何組合下又在深部加上一60度的高角度斷面；模擬結果顯示若在地下八公里深處斷層面由近水平角度轉折為60度可獲得較前述相當甚至更好的擬合結果，隱示山腳斷層可能不僅重新活化了造山時期的逆衝(底脫)斷層也同時活化了前造山時期正斷層系統。在全球許多後造山伸張環境的地震研究指出活動斷層斷面傾角急遽轉折處常為中大型地震的孕震發震位置，而2004年發生於台北盆地以東四獸山地區的中型地震其震源機制解與上述雙斷坡斷層幾何模型的深部斷坡相吻合，震源深度接近斷坪-斷坡轉折處，進一步支持此幾何的可靠性，並對大台北都會區的地震災害、以及台灣北部山脈垮塌轉型伸張的大地構造運動有重大意義。
並列摘要
The Taipei Metropolis, home to some 10 million people, is subject to seismic hazard from not only ground shaking in thick alluvial deposits due to distant faults or sources scattered throughout the Taiwan region, but also active faulting directly underneath. Northern Taiwan including the Taipei region is currently affected by post-orogenic (Plio-Pleistocene arc-continent collision) processes related to backarc extension of the Ryukyu subduction system. The Shanchiao Fault, an active normal fault outcropping along the western boundary of the Taipei Basin and dipping to the east, is investigated here for the areal extent and magnitude of its recent activity. Based on growth faulting analysis in the Wuku profile in the central portion of the fault, the Shanchiao Fault is found to be incessantly active since about 23 ka with an averaged tectonic subsidence rate about 3 mm/yr. A geologic profile across the north-central portion of the fault zone in the Luzhou area reveals similar main-branch fault half-negative flower structural pattern and slip rates observed in the Wuku profile, a phenomenon we interprete to originate from the geometry of the basin basement and the strong rheological contrast between unconsolidated basin sediments and basement rocks. One key horizon within the growth sediments – the top of the Jingmei Formation which was an alluvial fan formed rapidly when a major drainage reorganization occurred during the Last Glacial Maximum – is noted to serve as the marker of tectonic subsidence since its inception around 23 ka. A determination and compilation of the depths of the Jingmei Formation top horizon from nearly 500 borehole records within the Taipei Basin demonstrates that the hanging-wall of the Shanchiao Fault is deformed in a roll-over fashion with up to three branch faults sub-parallel to the main fault in the several-hundred-meter wide fault zone, and the offset is largest in the Wuku-Luzhou area in the central portion of the fault and decreases toward the southern tip of the fault. Along traces of the branch faults subtle fault-related geomorphic scarps can be mapped which exhibit a right-stepping en-echelon pattern, indicating recent sinistral transtensional faulting. Contemporary tectonic subsidence revealed by leveling data across the Taipei Basin during 1975 to 2003 was concentrated again in the Wuku-Luzhou near-fault hanging-wall area, probably representing an interseismic fault behavior. An attempt to resolve the poorly-known subsurface geometry of the Shanchiao Fault is carried out by simple elastic dislocation modeling of the surface deformation recorded by the Jingmei Formation top horizon compilation, which is representative of the latest Quaternary period as it spans probably more than 10 earthquake cycles. Preliminary results suggest that the Shanchiao Fault possesses shallow listric geometry where the low-dipping part may be inherited from the negative tectonic inversion of former thrusts, while deeper rift-related normal faults is also likely to be reactivated. Such constraints and knowledge are crucial in earthquake hazard evaluation and mitigation in the Taipei Metropolis, and in understanding the kinematics of transtensional tectonics in northern Taiwan.