探討泥岩區對臺灣西南部褶皺逆衝帶的高異常變形量之影響

Abstract

臺灣西南部位於歐亞板塊與菲律賓海板塊之交界處，為呂宋島弧與被動式大陸邊緣碰撞造成之造山帶，而臺灣西南部則位於臺灣造山帶與馬尼拉海溝之構造過渡帶。集集大地震後，對於斷層特性及活動性的調查更是目前評估地震災害最迫切的議題。臺灣西南部褶皺逆衝帶具有獨特的變形行為，由GPS資料可觀察到臺灣西南部確實為應變較大之區域，且速度場梯度變化極為明顯，但此區域卻是背景地震紀錄相對較少的區域，本研究區域為前陸盆地受造山帶荷重而沉降，高沉積速率使此區域沉積厚且廣的古亭坑泥岩，使臺灣西南部形成大範圍的惡地地形。本研究期望利用PSInSAR技術觀察臺灣西南部的地表變形，並結合GPS及水準資料對西南部的活動斷層做跨斷層的速度剖面討論斷層活動性，結果顯示臺灣西南部逆衝斷塊具有相當特殊的變形特徵，在龍船斷層及旗山斷層的上盤具約20 – 30 mm/yr的視衛星縮短量，不同於過去研究認為逆斷層上盤為相對抬升之區域，由大地測量資料皆顯示龍船及旗山斷層的下盤有較高之抬升速率。因此本研究將進一步利用DynEarthSol2D有限元素模擬討論此區域的變形機制，除了針對此區域的地下構造模擬斷層行為之外，亦討論此區分布廣泛的古亭坑泥岩可能存在的泥貫入體、泥基背斜及砂泥岩不同力學性質所帶來的影響。根據模擬結果顯示，本研究推論此區主控的變形機制為位於龍船斷層下盤的覆疊構造(duplex structure)及位於深度約4公里的向西傾斷層。除此之外，本研究亦發現古亭坑泥岩中的高壓層分布與PSInSAR觀察到的抬升區域位置有極高關聯性，而根據鑽井資料可得知高壓層位於龍船斷層西側的向西傾之斷層，因此認為此斷層因富含受壓流體而形成相對軟且易滑動的斷層面。因此本研究認為位於高壓層在臺灣西南部為一影響地表變形的重要因素之一，因其容易被觸發產生滑移，使得古亭坑泥岩此區域較不易累積能量而產生災害性的大地震，此推論亦符合GPS及歷史地震的紀錄。而本研究更進一步模擬2016年年初所發生的美濃大地震，討論其與地表淺層構造的同震滑移關係。

The variation of the anticline movements of the thrust sheets illustrates a distinctive deformation pattern in SW Taiwan. Thus I use the PSInSAR (Persistent Scatterer Interferometric Synthetic Aperture Radar) technique with constrains of precise leveling and GPS measurements to derive the line of sight (LOS) velocities in the study area and characterize the deformation patterns. I noticed that the maximum LOS velocities of ~ 20 - 30 mm/yr is recorded on the footwall of Lungchuan and Chishan Faults, which are considered as reverse faults in Gutingkeng Mudstone. The anomalous high deformation rate might be related to a ramp duplex located in the footwall of Lungchuan Fault. Or this deformation is related to the mechanical heterogeneity between mudstone and sandstone together with the mud-core anticline. I then use DynearthSol2D, an efficient unstructured finite element code, to simulate the deformation mechanism. In addition, we proposed that the high pressure zone in Gutingkeng Mudstone provides a weak zone which can easily be triggered by earthquake and produce coseismic slip. I attempted several hypotheses controlling high deformation rate to find the dominant deformation mechanism. I concluded that the main deformation mechanisms are the duplex structure underneath the Lungchuan Fault and the west-dipping fault with high fluid pressure in the west of the Lungchuan Fault. Besided, the co-seismic deformation from the 2016 Mw 6.4 Meinong earthquake shows the high correlation with the shollow structure, I would like to use the numerical modeling to address the mechanism how moderate earthquake could trigger the shallow structure by co-seismic displacement.