利用噪訊法分析台灣造山帶之非均向地殼構造

Abstract

藉由計算兩測站連續噪訊紀錄的交相關函數，可以得到波傳路徑間的經驗格林函數是二十一世紀初期地震學的重要發現之一。在過去十年來，這種顛覆過往認知，以表面波為主的新型資料被廣泛地應用於各種傳統地震學無法做到的研究，包含超高解析的地殼層析成像及偵測地底構造在時間上的細微變化等。得利於島上密布的測站及環海的豐富噪訊源，此法被應用於台灣的地震觀測網，並成功地導得高品質的短週期至中長週期(2-25秒)表面波。這些資料除了被用來建立高解析的表面波速度模型，探索因地震產生的構造變動，也被用以研究噪訊源特性。 本研究利用噪訊法擷取基頻表面波訊號，測量其群速度與相速度頻散曲線，並透過多尺度小波反演，建立了速度分佈圖。群速度反演結果顯示短週期波速與地表構造高度相關，在沿海與盆地區域呈現低速構造，而山區則為高速。與布蓋重力異常比較，可發現屏東重力低區與群速度中穩定呈現於多個週期的低速區吻合，皆反映了高屏地區厚層沉積物。然而，布蓋重力異常中最顯著的台中－埔里低區則未見於速度模型內，此結果顯示造成台中－埔里重力異常的來源可能在極淺或者深處，亦同時暗示可能有山根的存在。 本研究更進一步利用相速度資料反演震波的方位非均向性，研究結果顯示台灣地殼的方位非均向性隨著深度增加呈現顯著變化：淺層10公里內的快軸方向主要為東北-西南，平行台灣山脈走向；而下部地殼(>20公里)的快軸方向轉為東西向，平行板塊聚合方向，顯示在此深度受到高溫影響，岩石強度下降並反映隱沒板塊所造成的塑性剪切變形。此非均向性由淺至深漸變，約在十公里深處達到非均向性轉換邊界，暗示著地殼雖為分層變形，但上下層相互耦合，下部地殼對造山行為亦有貢獻。

We apply the ambient noise tomography (ANT) to Taiwan. In ANT, the path coverage is directly provided by the available inter-station paths. The high-density seismic stations in Taiwan thus offer a great opportunity for high resolution tomography with ANT. Besides two major seismic networks, Central Weather Bureau Seismic Network (CWBSN), and Broadband Array in Taiwan for Seismology (BATS), we have also incorporated the continuous broad-band data from three east-west linear arrays of the TAiwan Integrated GEodynamics Research (TAIGER) project. With above permanent and temporary seismic networks, we have achieved unprecedented path coverage of surface wave study in Taiwan. In particular, the unique geometry of TAIGER arrays allows us to largely improve the lateral resolution of the NNE-SSW Taiwan tectonic trend. We construct 2-D surface wave velocity maps form 4 to 20 sec using a wavelet-based multi-scale inversion technique. Patterns of lateral variations of our shorter period (<10 seconds) model demonstrate very good correlation with the surficial geology, whereas the overall structure, albeit with much better resolution in the shallow depth, is generally consistent with previously established body wave models. The absence of the source of Bouguer gravity anomaly in our model implying that it is likely caused by a deeper mountain root. We also investigate the crustal azimuthal anisotropy of Taiwan using seismic ambient noises. The reliability of the pattern of the resulting anisotropy is supported by the synthetic test and experiments of various azimuthal weighting schemes in 2-D. With iterative approaches, we then report 3-D seismic anisotropic tomography of Taiwan that shows a nearly 90° rotation of anisotropic fabrics across 10-20 km depth consistent with the presence of two layers of deformation. The upper crust is dominated by collision-related compressional deformation, whereas the lower crust, or the crust of the subducted Eurasian plate, is dominated by convergence-parallel shear deformation. This lower crustal shearing is interpreted as driven by the continuous sinking of the Eurasian mantle lithosphere when the surface of the plate is coupled with the orogen. The two-layer deformation redefines the role of subduction in the formation of the Taiwan mountain belt.