利用三維個別元素法模擬台灣西南部之地殼變形之研究

Abstract

本研究首先以中央地調所和中央氣象局所測量之GPS速度場資料來分析台灣西南部地區之地表斷層活動及地殼變形情形。根據GPS資料，研究區域中央靠近潮州斷層的地區之速度場朝幾乎正西方向；而研究區在西邊靠近海岸的區域，速度場則是朝西南的方向。速度場在屏東平原上，由東往西呈現一個逆時針旋轉之現象，推斷是由於屏東平原區域之構造逃脫現象所造成的。本研究亦使用GPS資料來分析研究區域地表之應變率。屏東平原上之應變率由北往南逐漸增大。此結果指出馬尼拉隱沒帶的增積岩體邊界為自由端，使西南部塊體可以朝西南方逃脫。 在以GPS分析完地表變形之後，本研究使用三維個別元素法來建立研究區域之地體構造模型，以電腦模擬之方式來分析造成研究區域地殼變形之原因。本研究所建立之研究區域之地體構造模型所包含之主要構造有基底滑脫面和旗山斷層、潮州斷層、高屏溪存疑性斷層等三條主要斷層，模型亦包含了變形前緣和縱谷斷層等兩個構造。模型西北部的北港高區及大陸棚區域之邊界條件被設定為固定不動；模型底部之邊界條件也被設定為固定不動；模型之東部邊界被視為菲律賓海板塊，邊界條件為菲律賓海板塊相對於歐亞板塊之移動速度，以每年7公分的速度朝北偏西50度前進。另一種模型東部之邊界條件，是將東部邊界視為台灣之中央山脈，以每年4.6公分之速度朝北偏西76度前進。以三維個別元素法模型所模擬之地表速度場結果可以看到類似於GPS速度場之逆時針旋轉之現象，但是沿海區域所模擬之速度方位角並沒有GPS速度場有那麼大的逆時針旋轉現象，故與實際觀測之速度方位角之殘差值較大。模擬結果並顯示出旗山斷層處於有逆衝及右移活動之狀態。

I use the GPS measurements which have been conducted by Central Geological Survey for investigating the fault surface activity and crustal deformation patterns in southwestern Taiwan. In the central study area near Chaochou Fault, GPS stations move nearly toward the west with respect to the stable continental shelf by using S01R as a reference point. In the coastal area of Pingtung Plain, the displacement vectors show a counter-clockwise deviation toward the southwestern, which we explain it in terms of lateral extrusion bounded by the major faults of study area. I also exploit the GPS data to calculate the strain rate in study area. The result of strain rate reveals a southward increasing of extension strain rate. This result infers that the lateral extrusion happen as a result of low confining boundary of accretionary prism of Manila subduction zone. Thus, I apply 3-D distinct element model to characterize the crustal deformation in study area with kinematic boundary conditions. I construct the model with six major discontinuities such as the basal décollement the Chishan Fault, the Chaochou Fault, the Kaoping Inferred Fault, the Longitudinal Valley Fault and deformation front, respectively. The northwestern boundary located on the Peikang High and the continental shelf is considered as fixed. The basal boundary condition is also fixed. The eastern boundary is assigned as the plate motion between the Philippine Sea plate and Eurasia with a velocity of 7 cm/yr toward N50°W. I assign another eastern boundary condition that I take the eastern boundary as the Central Ridge with a velocity of 4.6 cm/yr toward N76°W. The preliminary result of predicted velocity field of this model can reveal similar deformation pattern as shown by the GPS data. But the result of predicted velocity field in the coastal area of the Kaoping Plain has larger residual than that of other area. The result of model can also support the activity of the Chishan Fault.