利用接收函數模擬分析西藏中部地殼非均向性構造

Crustal anisotropy beneath the Hi-CLIMB array in Tibet from modeling receiver functions

黃奕翔(Yi-Xiang Huang)

指導教授 ： 洪淑蕙

國立臺灣大學/理學院/地質科學研究所/碩士(2014年)

https://doi.org/10.6342/NTU.2014.01356

摘要

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。

關鍵字

西藏高原 ； 接收函數 ； 地殼非均向性 ； 正演方法

並列摘要

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.

並列關鍵字

Tibetan Plateau ； receiver function ； crustal anisotropy ； forward modeling ； neighborhood algorithm

參考文獻

陳力維(2014)，臺灣地區近地表之震波非均向性研究，國立台灣大學地質研究所碩士論文.

梁軒碩(2012)，利用接收函數及表面波頻散之聯合逆推法探討西藏Hi-CLIMB陣列下方之地體構造，國立台灣大學地質研究所碩士論文.

Ammon, C. J. (1991), The isolation of receiver effects from teleseismic P waveforms. Bull. Seism. Soc. Am., 81, 2504-2510.

Yang, Y. , Chen, J. Y., Yang, X. S., Yan, X. B. and Zhang, G.L. (2010), Does alignment of melt enhance seismic anisotropy beneath Tibet. J. Geophys Res. doi: 1010.3969/j.issn.0253-4967.2010.01.006.

Aki, K., and K. Kaminuma (1963), Phase velocity of Love waves in Japan (part 1): Love waves from the Aleutian Shock of March 1957. Bull. Earthq. Res. Inst., 41, 243-259.