藉由地震波形模擬解析臺灣東北部琉球隱沒板塊的形貌與年齡約束

目前對於加瓜海脊西側的構造演化，學術界可歸納出兩種假說，其一認為花東海盆為被圍困在西菲律賓海盆西北方的岩體(125Ma)；另一則是推測花東海盆是西菲律賓海盆第二次張裂所形成 (35 – 50Ma)，其最大的分歧即是在於年齡定義得差異。前人對於此區域的年齡研究包含了古地磁模擬、雷利波相速度、熱流模擬以及氬-氬同位素定年。雖然大部分文獻同意花東海盆的生成年齡大約落在始新世，但現地採集的岩石進行地化分析卻將其生成年齡定義於早白堊紀時期。這些論據的分歧造成了花東海盆的年齡範圍從30Ma-125Ma，尚未有一套演化模型能統合解釋。本研究希望透過地震學的方式，利用振幅及波形的觀測得出一個板塊形貌，並透過與簡易板塊熱模型的擬合推估其板塊隱沒年齡，藉此提供花凍海盆生成年齡的約束。本研究收集臺灣寬頻地震觀測網(BATS)以及臺灣地體動力整合研究計畫(TAIGER)所記錄到以臺灣為中心震央距25度至90度的同心圓，地震深度100-1000公里的中、深部遠震波形資料。我們觀測要點為P波振幅比值的聚焦與散焦，以及因地震波沿著菲律賓海板塊隱沒而能於高頻波形所見的多重P波到時。我們透過參數調整出的隱沒板塊模型導入藉由GPU加速的有限差分法進行波形的模擬與計算，並建立一個高達6000筆參數組合的隱沒板塊模型庫。我們以不同的模型與觀測資料進行振幅比值與波形的擬合，再透過模型庫搜索得出最佳隱沒板塊模型。簡單板塊熱模型是基於Sleep (1973)理論，透過改變板塊隱沒年齡我們能得到不同的地溫模型，再藉由溫度與速度的關係式轉換得出P波速度模型。透過我們的最佳板塊模型與熱模型轉換的速度模型，以板塊速度剖面進行數值化差異性分析，我們便能得到剖面速度曲線與板塊年齡的關係。最後我們將得到的板塊模型結果套用至不同方位角的東北方地震，模擬結果與觀測結果相當吻合，藉此討論模型建立與側向變化的影響。本論文歸納出臺灣東北側的隱沒板塊年齡大約為50Ma，位於隱沒邊緣的花東海盆年齡勢必相去不遠，而早白堊紀年齡的板塊模型與觀測結果較不吻合。

關鍵字

板塊年齡；板塊熱模型；地震學；花東海盆；形貌學；隱沒板塊

並列摘要

There are two competing scenarios based on the tectonic evolution of western Gagua ridge, believing Huatung Basin (HB) is a trapped piece of an Early Cretaceous ocean basin (125Ma) or is formed in the second spreading era of West Philippine Sea Basin (WPB) (35 – 50Ma). Therefore, the age of the HB plays a critical role in this issue. Scientists endeavor in different aspects to constrain the precise age of HB which includes magnetic field simulation, seismology, thermal modeling and Argon dating. Although there is a general consensus that the birth of HB should be roughly in the Eocene, large uncertainties do exist among different studies ranging from 30 Ma to 125 Ma. In this study, we collected the tele-seismic waveforms with epicentral distance between 30 – 90 degrees and depth over 100 km recorded by the Broadband Array in Taiwan (BATS). We measure amplitude fluctuations in P-waves comparing to the PREM and characterize the multipathing arrivals caused by the subducting Philippine Sea plate (PSP) from high frequency waveforms. In order to investigate the detailed morphology of the subducting PSP, we use GPU-accelerated Finite-difference method to compute large number of the Green’s functions to create a library of the idealized 2D models. We grid search for six model parameters by minimizing the differences in amplitudes and waveform shapes between data and synthetics. The optimal slab model are compared to the velocity structure converted from the thermal model based on the theorem of Sleep (1973) to constrain the age of the PSP slab. After holding the optimal parameters provided from the result, we apply them to other events in the northeastern Taiwan. The comparison shows satisfactory solution while seismic wave propagating from different azimuths and directions. Afterall we evince that the slab age should be approximately 50Ma and no clear evidence for an older age possibility.

並列關鍵字

age ； geodynamics ； Huatung Basin ； morphology ； seismology ； subducting slab ； thermal modeling

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藉由地震波形模擬解析臺灣東北部琉球隱沒板塊的形貌與年齡約束

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