利用接收函數與移位成像探討臺灣之莫荷不連續面

Abstract

臺灣位於歐亞板塊與菲律賓海板塊的交界，碰撞與隱沒作用造成臺灣地區複雜的深部地殼構造。其中，地殼與上地幔分界的莫荷不續面分布是了解臺灣地體構造演化的重要觀察，然而過去的研究對於莫荷面深度的分布仍未形成共識。本論文利用兩種分析方法，一種是根據單一測站接收轉型波相的走時差以及振幅求解接收函數，並換算轉型波產生的深度;另一種則利用線性地震儀陣列，將接收函數波形移位成像(migration)至深度剖面。我們挑選了臺灣寬頻地震觀測網(BATS)中七個測站，以及臺灣大地動力學國際合作整合計畫 (TAIGER) 在臺灣中部以及南部所佈放的兩條地震站陣列，分析所記錄的震央距介於30至90度，規模大於5.5的遠震波形資料，求得各測站的接收函數，並觀察轉型波相(Ps)的到時與波形特徵來推算個別測站底下的莫荷面構造。同時，運用移位成像技術分析地震陣列記錄的數種散射波相，以反演陣列下方莫荷面的二維變化。 接收函數分析結果顯示，臺灣中部及東部的測站下方地殼內可能具有一個以上的不連續面，因此不易分辨出真正的莫荷面。移位成像的結果顯示莫荷面的劇烈起伏，亦暗示地殼內有不只一層不連續面。地殼內的複雜界面構造可能是導致各家研究結果差異的原因。本論文兩種分析結果均指出，臺灣莫荷面最深的位置在中央山脈下方。

We determine the depth variations of the Moho discontinuity beneath Taiwan from receiver functions and migration imaging. Taiwan is a young (~6.5 Ma) orogenic zone as a consequence of oblique collision between the Philippine Sea Plate and the Eurasian Plate. Much debate has centered on various models explaining the tectonic evolution of Taiwan orogeny. To that end, the depth of the Moho discontinuity is a key parameter for better understanding crustal deformation patterns associated with orogenic processes. In this study, we delineate the Moho depth variations by analyzing converted and scattered seismic waves. We analyze waveform data from teleseismic events recorded at the Broadband Array in Taiwan for Seismology (BATS) and temporary seismic stations of TGC-line and TGS-line of Taiwan Integrated GEodynamics Research project (TAIGER). We selected teleseismic events of magnitude larger than 5.5 and epicentral distance between 30 and 90 degrees. Our receiver function results show that there are likely more than one converted phases presenting within crustal depths beneath the stations we examine, suggesting the possible presence of multiple layering at least locally. Our migration images further reveal significant Moho topography across central and southern Taiwan, with the deepest Moho located under the Central Range. These results are generally consistent with the Moho location determined by 7.5 km/s contour in 3-D P-wave velocity model. Finally, we also observe a strong mid-crust discontinuity, perhaps associated with seismic anisotropy.