以觀測與理論波形之比較探討臺灣中部隱沒板塊的存在

Abstract

台灣位處歐亞板塊與菲律賓海板塊的碰撞帶，從菲律賓開始向東隱沒的歐亞板塊向北延伸至此處，隱沒板塊在台灣中部的存在與否及其可能的特徵是了解區域地體構造的一個重要議題。為了解隱沒的歐亞板塊是否延伸至台灣中部，本研究以三維震波模擬的方法來定量地探討隱沒板塊可能具有的特徵，如傾角以及波速變化等對遠震P波波形的影響。我們使用台灣大地動力學國際合作研究(TAIGER)計畫中的寬頻遠震資料，選取其中沿南北方向縱貫臺灣佈設的寬頻測站，採用譜元素法(SEM)計算多種不同速度模型中的理論波形，分別測量模擬與觀測波形的P波到時。藉由不同模型中理論波形所測量的到時的變化，來探討包含地形、地殼結構以及不同隱沒板塊的速度模型對P波到時的影響，並通過與實際記錄測量的到時的比較，找出最符合觀測資料的隱沒板塊速度模型，由此判斷在台灣中部地殼下方隱沒板塊存在的可能。 對包含地形、地殼速度模型的模擬結果顯示，在本研究所使用的頻段範圍(週期15秒或更長)，台灣地區淺部地層對到時的影響較小；來自東南方的遠震結果顯示，相較於台灣北部，隱沒板塊的存在使P波在台灣中部及南部的到時較早，其中較陡的傾角及速度差異較大的隱沒板塊，使在台灣中部到時提早的較多；來自西南方的遠震到時變化均較來自東南方的遠震小。綜合所有模擬結果並與觀測比較可以看出，當台灣中部地殼下方的隱沒板塊向東傾斜30

The Taiwan Island is a result of the convergence between the Eurasia and Philippine Sea plates. The east-dipping Eurasia slab extends from the Philippines to the Taiwan Island. The northernmost reach of the Eurasian slab beneath Taiwan and the characteristics of the slab are important issues for understanding the geodynamics of Taiwan’s regional tectonics. To examine whether the east-dipping Eurasian slab extends northward beneath central Taiwan, in this study, we invoke the 3D seismic wave modeling to calculate the synthetic seismograms to explore the possible effects of the slab. For example, the effect of the dip angle of the slab and its P wave speed perturbation on the teleseismic P waveforms. The TAiwan Integrated GEodynamic Research (TAIGER) project deployed several well-designed temporary arrays, and the broadband teleseismic data from the stations along a north-south transect across Taiwan are utilized here to examine the patterns of the P waveform variations. We use SEM to calculate synthetic seismogram for different models, and measure the P wave arrival times. These model-predicted P wave arrival times are used to study the effect on P wave arrival times by topography, crustal structure and different slab models, and compared with P wave arrival times from records. Comparison between observed and predicted arrival times helps us to determine the existence of subducted slab under central Taiwan. Modelings with topography and crust show that shallow structures have a small effect on P wave arrival times in Taiwan in the frequency range in this study (period longer than 15 sec). Results for models with slabs for earthquakes in the southeast show that in central Taiwan the P waves arrive earlier relative to northern Taiwan, and the slabs of larger dip angle and/or larger velocity perturbation result in earlier arrival times in central Taiwan. Results for the slab models for the earthquake in the southwest have smaller variation in arrival times relative to earthquakes in the southeast. Combining all the results and comparing to observations, the slab model with a 4% velocity perturbation, results in predicted arrival times closest to the observations. Therefore, our results point to the existence of subducted slab beneath central Taiwan.