- 學位論文
利用噪訊成像法探討北沖繩海槽地殼速度構造
Ambient noise tomography for the crust of the northern Okinawa Trough
摘要
本研究使用2018年9月至2019年6月布放於北沖繩海槽的海底地震儀陣列資料，以噪訊成像技術建構該區的三維震波速度模型。首先，透過垂直分量連續紀錄的互交相關函數擷取雷利波的經驗格林函數，並量測其的相速度頻散；接著， 用4—30秒的頻散資料進行以小波為基底函數的多重尺度反演，藉此得出北沖繩海槽第一個地殼尺度的Vs和其方位非均向性的三維模型。 Vs模型的速度分布與該區域二維震測剖面和有限頻寬三維震波成像之初步結果大致吻合，在弧後區域展現較快的Vs，弧前和琉球島弧則為較慢的Vs。方位非均向性的快方向在淺層弧前區平行於海溝，有可能為弧前沉積物受當地應力而變形所致。在模型的較深區，中地殼的快方向大致平行於海溝，下地殼區的快方向則轉向至垂直於海溝走向。快方向平行於海溝的非均向性可能為地殼內的非均向性礦物受到琉球島弧向南移動形成的右移剪切力影響，導致其排列方向平行於海溝走向。下部地殼靠近上部地函，其非均向性的快方向轉向垂直於海溝，則可能導因於地函楔角流(corner flow)運動。
並列摘要
From September 2018 to June 2019, an array of broadband OBSs was deployed in the north Okinawa trough (NOT) and its neighboring sea as part of the Taiwan-Japan collaborative project led by Institute of Earth Sciences, Academia Sinica (IES), Taiwan Ocean Research Institute (TORI), and Japan Agency for Marine-Earth Science and Technology (JAMSTEC). Continuous vertical component data from OBSs, F-net stations, and six temporary stations were used to extract the empirical Green’s functions of Rayleigh waves. With the measured dispersion (4-30 s) data from the noise-derived Rayleigh waves, we implemented a one-step, wavelet-based multi-scale tomographic technique to invert for 3-D models of both Vs and its azimuthal anisotropy of the crust and the shallow mantle. Major features in our model are generally consistent with multi-channel seismic 2-D and finite-frequency 3-D models, showing higher Vs associated with the OT and lower Vs in the forearc region. Trench-parallel fast direction of azimuthal anisotropy appears in the forearc shallow crust, most likely caused by the deformation fabrics due to rifting. The mid- to lower crust of the NOT was dominated by trench-parallel anisotropy, probably reflecting a prevalence of sub-vertical shear plane (foliation) resulting from the rifting-shearing processes associated with Okinawa trough opening and southward propagating. In the shallow mantle, the anisotropy fast direction normal to the trench correlate well with the corner flow dynamics in the mantle wedge. As a result, both shape preferred orientation (SPO) and lattice preferred orientation (LPO) were involved to create the overall pattern of anisotropy in the study region.