龍門山斷層帶南段天全河沿岸河階的活動構造特性研究

Abstract

龍門山斷層帶位於中國青藏高原與四川盆地之間，這個交界帶一直以來都被地質學家認為是構造活躍的區域，並且在近年來發生了兩起重大的地震。首先是2008年發生在龍門山斷層帶北段的汶川地震，此次的地震在地表上有大規模的破裂帶與錯移。五年後，發生在龍門山斷層帶南段的蘆山地震，卻沒有明顯的地表破裂。蘆山地震發生前，地質學家對這個區域的活動構造研究較少，所以到目前為止，蘆山地震究竟是由哪一條斷層所造成，還是有其爭議。 本研究利用構造地形學的方法，研究龍門山斷層帶南段河階之特性、年代與成因，來推測此地區的構造發育與其影響因素。首先藉由數值地形模型，調查蘆山地震區域的河階分布狀況，在天全河沿岸的天全及始陽兩個城市，發現有大規模階地群的分布。經由野外調查及測量之後發現，兩地的河階均僅有約1-3公尺厚的沉積層蓋在砂頁岩互層的底岩之上，沉積層中主要為與現今河道中大小及來源相似的礫石。透過利用RTK-GPS測量階地與河道之坡度，發現兩者的坡度相似。這些證據均顯示兩地的河階可能屬於被構造所抬升的底岩型河階。此外，RTK-GPS的河階測量數據搭配DEM的分析，可以更有效判斷兩地的河階邊界與階面，結果顯示天全縣河階數約有4階，而始陽鎮則多達11階。本研究並利用天全縣河階的定年結果，計算Terrace 1形成後區域的下切速率約為介於1-2 mm/year，這個速率相似於龍門山斷層北段汶川區域的構造活動速率，顯示龍門山斷層帶南北段近期的構造活動速率可能沒有太大的差異。此外由定年結果也發現近期天全縣構造速率變化有由慢轉快的趨勢，與前人青衣江河階之研究相似，因此可能表示著整個龍門山斷層帶南段近期的構造速率皆有由慢轉快的趨勢。 根據本研究的結果，顯示這個區域的構造活動對人類社會具有潛在的未來威脅，因此本研究成果將有助於該區域未來進一步的新構造研究，並且用於預防地震及相關地質災害，以保護該區域高達百萬人的生命及財產安全。

The Longmenshan fault zone at the western edge of the Sichuan Basin has long been identified as an active tectonic belt. This has been clearly illustrated by the disastrous Wenchuan and Lushan earthquakes in the recent decade. The two earthquakes, however, have distinctive characters. In the north, the Wenchuan event occurred on major fault zones identified previously. But in the south, the Lushan event was not accompanied by surface ruptures. Before the Lushan event, there were not many active tectonic studies in this area, either. As a result, the seismogenic structure of the Lushan event is still under debate. In order to further understand the neotectonic characteristics of the Lushan earthquake region, we analyzed fluvial terraces first by satellite images, in the hope that such geomorphic features would provide information of active structures of the area. Along the Tianquan River, river terraces are particularly well developed near two cities, Tianquan and Shiyang. After detailed field investigations, we found that underneath these terraces, early Tertiary bedrocks crop out below river sediments that are only 1-3 m thick. This indicates that the Tianquan River has incised into bedrocks. The slope of the terrace surfaces is similar to that of the present-day riverbed, and the river sediments in the terrace outcrops have similar grain size distribution as current riverbed sediments. Therefore, we suggest that the terraces along the Tianquan River were result of tectonic uplift. Using the Real Time Kinematic-Global Positioning System (RTK-GPS) and Digital Elevation Model (DEM) results, we defined Tianquan terraces with 4 levels, and Shiyang terraces with 11 levels. With the ages of the terraces and the amount of incision, we obtained the incision rate of the Tianquan River. The incision rate of the Tianquan River is about 1-2 mm/year, very similar to the structural activity rate of the northern Longmenshan fault zone. We also found the structural activity rate in Tianquan area increased recently, and this trend is very similar to the results obtained from studies of Qingyi River terraces, in the southern region Longmenshan fault zone. Therefore, we speculated that the structures became more active recently in the entire southern Longmenshan fault zone. Based on our results, the tectonic activity poses potential threatening to this area. Therefore, we hope the results of this study would provide more information of neotectonic characteristics of the southwestern Sichuan Basin, as well as future earthquake hazards in this densely populated region.