1999集集地震引致的河道遷急點倒退速率與倒退機制

Abstract

1999集集地震係由車籠埔斷層活動所引起，該斷層為一南北走向、向西逆衝之斷層。車籠埔斷層地表破裂線貫穿中部地區大部分的河道，由於斷層破裂帶抬升，而於各河道形成遷急點，因為西部麓山帶之地質年代極為年輕，出露之岩層膠結不良、強度低，在水流的作用下會產生嚴重的沖蝕，目前部分河道遷急點已有明顯且大量的上溯情形。交通大學(2012)的研究結果顯示，大安溪隆起段之遷急點在93年之後倒退速率約為每年數十公尺至數百公尺，此倒退速率相當驚人。 本研究以正射影像或航空照片觀察河道遷急點倒退的情形，並估算倒退速率，再配合現場調查資料，建立河道遷急點倒退機制等。研究結果顯示倒退機制可區分為四類，即平面滑動破壞、倒懸破壞、磨蝕下切以及塊體抽離機制。倒退率受水流流量、遷急點的落差與坡降、岩體強度的影響，大安溪平均每年倒退73.1公尺，與國外每年數公釐至數公分相比，倒退量甚鉅。 本研究建立集集地震造成遷急點之歷年倒退資料目錄，利用交通大學(2012)、Hayakawa and Matsukura (2003)等遷急點倒退率經驗迴歸式進行迴歸測試，公式中考慮因子包含水流流量、遷急點的落差與坡降以及沖蝕指數Kh等因子，結果顯示有幾個遷急點實際倒退速率大於回歸出來的倒退速率，推測為倒退機制的差異造成。

The Chi-Chi earthquake occurred on 21 September 1999. This earthquake produced surface ruptures of 100 km in length along the Chelongpu fault, which strikes in north-south direction. The knickpoints formed at the river channel owing to the uplift of the Chelungpu fault. The bedrock of the knickpoints belongs to the geological provinces of the Western Foothills. The Western Foothills are composed of the Late Cenozoic sedimentary rock which is usually weak and poorly cemented. Severe incision in the weak bedrock was activated after its armor layer was removed. The maximum annual rate of kninkpoint retreat reached hundreds of meters. Based on aerial photographs/orthophotographs, derived Digital Elevation Data, cross-section surveyed data, and field investigation, morphological changes and retreating processes of the kninkpoints were evaluated. The mechanisms of the knickpoint retreat were concluded into four types: 1) plane sliding; 2) overhang falling; 3) abrasive erosion; 4) block plucking. The conceptual models of the knickpoint retreat were also proposed in this study. Two empirical formulas of the knickpoint retreat rate were verified with the knickpoints data. The results suggest that the empirical formulas might be not suitable for cases of knickpoints caused by earthquake uplift, which could be improved by considering the mechanisms of knickpoint retreat.