槽溝開挖與古地震研究計畫(III)
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2004/01-2004/12
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台灣島是受到菲律賓海板塊上的火山島弧與歐亞大陸發生碰撞形成的島嶼，因此在遭受強烈的擠壓作用下，產生許多東北走向的逆斷層與東西走向的平移斷層以及褶皺構造。這些構造活動的同時也引發許多大大小小的地震，而在大地震發生的同時經常伴隨著地表破裂，過去百年來台灣島就有數次的斷層活動，並造成地表破裂，如1906年嘉義大地震產生的梅山斷層、1935年中部大地震的屯子腳斷層、獅潭斷層與神卓山斷層，1946年新化地震的新化斷層，1951年花東大地震的米崙斷層、玉里斷層與池上斷層，以及1999年集集地震的石岡斷層、車籠埔斷層與大尖山斷層。數百萬年以來台灣島就在無數次的地殼變動下持續的擴張與升高。從近百年來規模大於7.0的地震的共有7次，就可以了解台灣是世界上地震最為活躍的地區之一。另外現今台灣的人口密度約650人/平方公里，也是了解到當大地震發生時可能造成的傷亡與災害，也是最為嚴重區域之一。近年來台灣經濟急速發展，大規模開發土地，迫使居住空間逐漸擴及活動斷層沿線。因此「活動斷層位置的精查」、「斷層活動性研究」以及「地震災害研究」等工作更是迫在眉睫。雖然目前針對地震的發生時間、地點與規模都無法做準確的預測，但是近三十年來地質學家都希望能經由槽溝的挖掘尋找古地震證據，以進一步了解地震成因與斷層的構造特性與活動時間，並以此推估地震的週期或大地震的發生機率。因此，槽溝開挖調查是古地震研究中最重要以及最直接的方法。除了槽溝開挖調查之外，活動構造形成變動地形的特性也是研究長期的斷層活動性的另一重要方法，尤其河階地形更可以讓我們深入了解長期性與區域性的構造活動（活動斷層與活動褶皺）。因此，本研究首先針對中央地質調查所所訂定的其中25條活動斷層作先期的古地震研究，以及全省的河階地形建置，依照計畫規劃進度分為五年期研究。九十三年至九十五年之斷層調查順序調整則遵照九十二年第二次工作協調會之決議辦理。 第一年：新城斷層、新化斷層、車籠埔斷層、梅山斷層、彰化斷層。 第二年：九芎坑斷層、觸口斷層、大尖山斷層、後甲里斷層、六龜斷層。 第三年：山腳斷層、大平地斷層、湖口斷層、米崙斷層、月眉斷層、玉里斷層。 第四年：獅潭斷層、屯子腳斷層、神卓山斷層、池上斷層、奇美斷層。 第五年：三義斷層、大甲斷層、鐵砧山斷層、鹿野斷層、利吉斷層。
The subduction of the Eurasian continent beneath the Philippine Sea plate is represented by the westernmost thrust of the Changhua fault in the Western Foothills. The Western Foothills consist of a series of subparallel thrust faults and related folds which form a west-verging fold-thrust belt. Fault-bend and fault-propagation fold structures are also common in the Western Foothills. The thrust system in northern Taiwan is now inactive with normal-faults commonly reactivating former reverse faults throughout the late Quaternary. The Western Foothills and Longitudinal Valley are a strongly active tectonic area. The most conspicuous tectonic features in those areas are north-trending thrust faults. This study will be focued on several active faults in the northern and eastern Taiwan, such as the Shanjiao,Dapingdi, Hukou, Milun, Yuemei, and Yuli faults. Recent uplift of these thrust sheets results in river incision and development of complex fluvial terraces. Several lateritic and non-lateritic terraces, for example, are preserved along the Taan and Tachia Rivers. Analysis of nearby fluvial systems provides a proxy for thrust deformation because localized long-term uplift produces anomalous changes in river systems. The purpose of this project is to discuss how fault activity can be used to show the tectonic implications. In this study, we focus on the spatial variation of slip rate and activity based on the trenching study along the activity fault. We identify the surface rupture deformation and propose possible tectonic interpretations. In addition, we will reconstruct the characteristics of the river terrace in the eastern Taiwan.
The subduction of the Eurasian continent beneath the Philippine Sea plate is represented by the westernmost thrust of the Changhua fault in the Western Foothills. The Western Foothills consist of a series of subparallel thrust faults and related folds which form a west-verging fold-thrust belt. Fault-bend and fault-propagation fold structures are also common in the Western Foothills. The thrust system in northern Taiwan is now inactive with normal-faults commonly reactivating former reverse faults throughout the late Quaternary. The Western Foothills and Longitudinal Valley are a strongly active tectonic area. The most conspicuous tectonic features in those areas are north-trending thrust faults. This study will be focued on several active faults in the northern and eastern Taiwan, such as the Shanjiao,Dapingdi, Hukou, Milun, Yuemei, and Yuli faults. Recent uplift of these thrust sheets results in river incision and development of complex fluvial terraces. Several lateritic and non-lateritic terraces, for example, are preserved along the Taan and Tachia Rivers. Analysis of nearby fluvial systems provides a proxy for thrust deformation because localized long-term uplift produces anomalous changes in river systems. The purpose of this project is to discuss how fault activity can be used to show the tectonic implications. In this study, we focus on the spatial variation of slip rate and activity based on the trenching study along the activity fault. We identify the surface rupture deformation and propose possible tectonic interpretations. In addition, we will reconstruct the characteristics of the river terrace in the eastern Taiwan.