利用接收函數探討北越Song Ma斷層帶地殼構造

在近代對於地殼構造、厚度以及莫荷面深度的研究，有不少是利用接收函數(Receiver functions, RFs)的方法去獲得。東亞地區是印度板塊、歐亞板塊、菲律賓海板塊以及太平洋板塊之間互相作用之區域，因此造成東亞地區複雜的地體構造與地表變形。印度板塊和歐亞大陸板塊約在第三紀(Tertiary)左右開始碰撞，使西藏高原因為岩石圈增厚而抬升，並推動中南半島。是故，在北越Song Ma斷層剪切帶其區域應力特性及地體構造與上述此區域的板塊擠壓碰撞有密切關係。在目前的文獻中大多數探討區域多是集中於北越紅河斷層帶(Red river fault zone)之鄰近區域與Song Ma斷層帶西北段，而對於Song Ma斷層帶東南段這部分之地下構造上仍處於不明瞭的情況。為了突破此情況，國立中正大學於2009年與越南地質科學院合作在北越Song Ma斷層剪切帶東南段設置了12部臨時的寬頻地震儀器，由於在此區域目前所監測的地震大多發生於地殼淺部，地殼深部的構造則受限於地震數量稀少使得對於深層的地下構造不甚了解。為了解決這個問題，本研究利用遠震接收函數來解析深層的地下構造、莫荷面深度以及各層非均向性之特性與方向。初步結果顯示了此區之莫荷面深度介於26~35公里深之間，S波速度值介於4.1~4.7( km/sec)。在非均向性結果的部分大致可分類成兩類，一類是在0~20公里深的地殼中非均向性慢軸的部分，慢軸方向結果顯示大致垂直斷層構造；另一類是深於20公里下部地殼及上部地函中非均向性快軸的部分，其快軸方向結果顯示大致平行於在270至240個百萬年前印度支那地塊(Indochina block)向北移動與南中國地塊(South china block)發生碰撞的方向。由以上結果可讓我們更進一步瞭解Song Ma斷層帶東南段及其鄰近區域的地體演化。

關鍵字

北越；剪波分裂；接收函數；非均向性

並列摘要

This pilot study analyzes earthquake data recorded by the broadband seismic network deployed along the Song Ma fault, northern Vietnam. We have selected teleseismic events with Mw≥5, and epicenter distance is between 30o and 90o. A Multiple-Taper Correlation (MTC) method is adapted to calculate receiver functions (RFs) for each station. The converted phase, such as P-to-S obtained from RFs, allows us have insights on the characteristics of crustal structures including the dip of discontinuous interface and anisotropy as well. The above properties have great effects on amplitudes and arrival time of RFs. Thus, we have applied an initial anisotropic velocity model to obtain synthetic RFs. Furthermore, we used a Neighborhood Algorithm to search an optimum model which had minimum misfit between the observed RFs and synthetic ones. Our preliminary results indicate that the depth of the Moho discontinuity in the Song Ma fault zone is between 25 km and 35 km, and the S-wave velocity is from 3.6 km/s to 4.5 km/s.

並列關鍵字

North Vietnam ； shear wave splitting ； receiver function ； anisotropy

參考文獻

Bai, L., Tian, X., Ritsema, J., 2010. Crustal structure beneath the Indochina Peninsula from teleseismic receiver functions. Geophysical Research Letters 37, L24308. doi:10.1029/2010GL044874.

Bianchi, I., Piana Agostinetti, N., De Gori, P. & Chiarabba, C., 2008. Deep structure of the Colli Albani volcanic district (central Italy) from receiver function analysis, J. geophys. Res., 113, B09313, doi:10.1029/2007JB005548.

Brocher, T. M. and Christensen, N. I., 1990. Seismic anisotropy due to

preferred mineral orientation observed in shallow crustal rocks in

Cassidy, J. F., 1992. Numerical experiments in broadband receiver function analysis, Bull. Seism. Soc. Am., 82, 1453-1474.

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利用接收函數探討北越Song Ma斷層帶地殼構造

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