- 學位論文
西藏地區之多尺度有限頻寬噪訊雷利波速度層析成像
3-D Multi-scale Finite-frequency Ambient Noise Rayleigh Wave Tomography of Crustal S-Wave Velocity Structure beneath Central Tibet
摘要
研究地球地殼與上部地函的S波速度構造有效且廣泛運用的方式就是利用表面波走時層析成像進行分析。傳統上不論是基於波線理論或考慮二維的有限頻寬散射，都先透過建立不同頻率下的二維相速度，再利用相速度對深度上的一維敏感度求解三維的S波速度。然而，此兩階段式的分析方法，無法合理考慮有限頻寬的震波傳遞經過側向不均質的地球產生的三維散射效應。因此本研究基於有限頻寬下的單一散射理論計算各頻率各資料的相位差敏感算核(kernel)，針對基頻的雷利波，直接進行西藏中部的三維S波速度構造層析成像。 本研究的資料來自2004年至2005年在西藏中部地區佈置的Hi-CLIMB地震測站陣列。透過一般的周遭噪訊方法，利用兩兩測站連續記錄得到交互相關函數，與藉由西藏一維速度參考模型所擬合之波形用多重濾波方法(multi-taper method)量取相位差，再運用理論計算的敏感算核逆推三維的S波速度變異。我們使用奇異值分解(SVD)進行逆推，並引入小波基底轉換的參數化方式達成基於資料密度的多重尺度解析能力，以避免資料密度在空間上分佈不均的問題。 我們的結果顯示以班公怒江縫合帶為分界，整個地殼與上部地函的速度構造呈現北慢南快的分佈，而介於班公怒江縫合帶和印支雅魯藏布縫合帶之間的拉薩地塊，則呈現明顯較快的速度。本研究結果中，並沒有呈現明顯且互相連接的下部地殼低速構造，這暗示著研究區域下並沒有顯著支持西藏地區下地殼流動模型的證據。未來若加入更多的真實地震長週期資料，可對於西藏-喜馬拉雅造山帶的構造運動模型提出更好的解析能力。
並列摘要
Surface wave travel-time tomography has been widely used as a powerful strategy to image shear wave velocity structure of the Earth’s crust and upper mantle. Traditionally with either ray theoretical great-circle approximations or 2-D phase kernels, phase velocity maps are first obtained at multiple frequencies. They are then combined to invert for shear wave velocity structure using 1-D depth-varying Frechet derivatives of phase velocity with respect to shear wave speed. Such approach runs short on considering the directional- and depth-dependence of scattering while surface wave propagating through laterally heterogeneous Earth. We here present a fully 3-D finite-frequency method based on the Born scattering theory in conjunction with surface-wave mode summation and apply it to regional fundamental Rayleigh wave tomography in central Tibet. Our data were collected from Hi-CLIMB array in the central Tibet during 2004-2005. Following a standard procedure to obtain empirical Green’s functions of Rayleigh waves from ambient noise cross correlation functions (CCFs) between station pairs, the phase differences between the CCFs and corresponding synthetics are measured by a multi-taper cross-spectral method. We apply the 3-D sensitivity kernels at individual frequencies convolved with the same eigentapers used in the phase measurement to conduct a 3D tomography of shear wave velocity perturbations with respect to a spherically-symmetric earth model suitable for central Tibet. A wavelet-based, multi-scale parameterization is invoked in the tomographic inversion to deal with the intrinsic problem of unevenly distributed data and resolve the structure with data-adaptive spectral and spatial resolutions. The result shows that the crust is generally slower to the north of the Bangong-Nujiang Suture (BNS) in marked contrast to the south with higher speeds. The absence of pervasive low velocity anomalies in the mid-to-lower crust indicates that the ductile channel flow of the lower crust may be inactive beneath southern Tibet. The model resolution in the lithospheric mantle can be improved by integrating longer-period surface data from distant earthquakes, which will yield better constrains on the geodynamic process of the Himalayan-Tibetan orogeny.
參考文獻
延伸閱讀
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