西藏地區具有世界上最厚的地殼，並記錄下所經歷過的造山運動資訊，一直以來都是科學家關注的區域，藉由分析與討論西藏地區的非均向性構造，以了解地下地層在造山運動過程中受到的應變與應力情形。西藏高原由數個地塊所組成，其中主要的構造包括南邊的拉薩地塊和北邊的羌塘地塊，並以班公怒江縫合帶為界。南北兩地塊在地球物理性質和地質條件上有很大的差異，暗示各自經歷了不同的地質事件。本研究使用在2004年到2005年間架設於西藏中部的Hi-CLIMB (Himalayan-Tibetan Continental Lithosphere During Mountain Building)寬頻地震陣列進行資料處理，分析接收函數隨後方位角的變化，並以此大量地建構非均向性速度模型，透過最佳化隨機搜尋法進行理論合成接收函數和觀測資料的波形擬合，找出西藏中部底下的地殼非均向性分布以及特性。除了波形的擬合之外，我們還加入了簡諧函數與非均向性造成波形變化特徵，試著共同分析西藏中部地區的地殼非均向性。 本研究利用正演接收函數模擬方法解出較可信的非均向速度模型，發現中部地殼出現較強的非均向性地層通常和低速構造是並存的，然而該地層中的非均向性強度無法單純以礦物晶格優選排列來解釋，可能該地層中的非均向性和低速構造是由部分熔融的地殼塑性流所造成。而淺部地殼和中部地殼明顯不同的非均向性質亦暗示成因的差異，淺部地殼的非均向性可能主要與近地表地層中未癒合且高角度的裂隙有關。

It is well known that the India-Eurasia collision beginning at about 55 Ma ago results in the world’s highest Himalayan mountain and largest Tibetan Plateau with an average elevation of 5 km and crustal thickness of 70 km. Strain-induced seismic anisotropy is commonly used as a proxy to map the distribution and characteristics of lithospheric deformation. In this study, we focus on investigation of the crustal anisotropy beneath central Tibet by means of modeling radial and transverse receiver functions (RFs) observed along a N-S striking, 800-km long linear array from the passive seismic experiment of Project Hi-CLIMB.
To quantitatively characterize the depth-varying anisotropic structure in the crust under the plateau, we combine azimuth-dependent RFs in both radial and transverse components to invert for 1-D layered anisotropic velocity models under individual stations. As the inverse problem is highly non-linear with numerous unknown model parameters, we choose a neighborhood algorithm to search for the solution with the global minimum of the optimized function, defined as the decorrelation coefficient between observed and synthetic RFs.
The resulting models show that the uppermost crust at depths less than 10 km beneath most stations reveals >10% strong anisotropy and a nearly vertical fast symmetric axis with plunge greater than 70o. For the stations near the E-W trending suture zones between terranes, the azimuth of the obtained fast axis is approximately aligned E-W, while those within the Lhasa and Qiantang terrane the fast axis is oriented more irregularly. These characteristic features suggest that the sutures and E-W striking strike-slip and N-S striking normal faults randomly distributed within the terranes may be attributed to the observed pattern of upper crust anisotropy. A strong anisotropic layer with the subhorizontal symmetric fast axis is observed in the middle crust at the depths of 20-35 km, collocated with the low shear velocity zone. The presence of a low-viscosity, ductile channel with preferred alignments of partial melt inclusions and/or anisotropic mica minerals may be attributed to the observed anisotropy in the middle crust. The anisotropy in the lower crust is generally less well constrained due to the low signal-to-noise ratios of traverse-component RFs as well as the P-to-s conversions at the velocity discontinuities in the lower crust strongly influenced by complex shallow structures.

口試委員會審定書 #
摘要 1
Abstract 2
目錄 4
表目錄 10
第1章 緒論 11
1.1 引言 11
1.2 非均向性 13
1.2.1 震波非均向性研究 13
1.3 研究區域背景 19
1.4 前人研究 20
1.5 研究動機及目標 26
第2章 地震資料 28
2.1 研究區域和測站分布 28
2.2 地震和波形資料篩選 29
第3章 研究方法與原理 32
3.1 Ps 轉換波相 33
3.2 接收函數( Receiver function ) 35
3.3 接收函數之計算 37
3.4 地震資料的疊加 40
3.4.1 波線參數修正 ( Moveout correction ) 41
3.4.2 疊加區域的選取 41
3.5 接收函數正演模擬 43
3.5.1 鄰近演算法 44
3.5.2 誤差函數 45
3.5.3 定義變數空間 46
3.6 速度構造對接收函數的影響 48
3.6.1 側向均質且水平地層構造的接收函數 48
3.6.2 非均向性對稱軸呈水平向地層構造的接收函數 49
3.6.3 非均向性對稱軸呈垂直向地層構造的接收函數 50
3.6.4 非均向性對稱軸傾斜之地層構造的接收函數 51
3.6.5 傾斜速度不連續面構造的接收函數 51
3.7 初步辨識非均向性與傾斜速度不連續面構造 58
3.7.1 單一非均向性水平地層或均向傾斜的不連續面構造 58
3.7.2 複合構造 61
3.8 初始速度模型 68
3.8.1 初始速度模型測試 68
3.8.2 建構初始速度模型 70
第4章 研究結果 75
4.1 拉薩地塊南段(H1010-H1200) 78
4.2 拉薩地塊北段(H1210-H1400) 79
4.3 羌塘地塊(H1410-H1630) 79
4.4 非均向性速度模型沿南北側向的變化 88
第5章 討論 93
5.1 簡諧函數模擬Pms波到時變化 93
5.2 正演接收函數模擬方法對速度模型的約束能力 103
5.2.1 鄰近地層非均向對稱快軸方向trend相差180ｏ 104
5.2.2 非均向性對稱軸的傾角接近垂直 106
5.3 模型回復能力 107
5.3.1 單層非均向性介質回復 107
5.3.2 多層非均向性介質回復 110
5.4 地殼非均向性特徵 118
第6章 結論 124

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