歐亞板塊在南中國海地區沿著馬尼拉海溝往東隱沒至菲律賓海板塊底下，其板塊隱沒的深度亦向東邊逐步遞增。由於隱沒板塊溫度相對周圍地函環境較低，呈現較高的地震波速；而其上方之楔形地函則因發生部分熔融作用而使得地震波速相對較低，因此預測在垂直馬尼拉海溝方向上有側向的震波速度變化。本研究分析地震震源分布於菲律賓群島兩側，由台灣的寬頻地震網（Broadband Array in Taiwan for Seismology, BATS）所記錄之表面波的頻散曲線，逆推一維速度構造，來探討垂直馬尼拉海溝方向的側向速度變化。 表面波的頻散意指不同週期的波會有不同的波速，本研究採用多重濾波法（Multiple Filter Technique, MFT），求取雷利波的群速頻散曲線；以AK135為初始模型，利用阻尼最小平方差方法（Damped least square solution）逆推最符合觀測頻散曲線的一維速度構造。逆推所採用之阻尼係數由合成震波之頻散曲線逆推，格點搜尋得到最佳之阻尼係數；逆推過程使用Herrmann提出的程式surf96對觀測震波的頻散曲線進行逆推求得最佳的一維速度模型。 由頻散曲線與逆推速度模型的比較可以得知，在垂直馬尼拉海溝方向的側向速度變化中，東側路徑之地層波速整體多高於西側；而西側中A地震之路徑之低速異常可能由於該路徑方向上的構造變化導致，B路徑則經過火山系統而相對低速，東側之C整體波速比西側高，D地震到西側測站之路徑速度略低，E路徑為單純海洋性板塊而速度明顯高於其他路徑；在所有地震到單站的比較中，B、C與D地震路徑經過或接近火山系統而相對低於A與E地震，其中E路徑大部分為海洋性板塊而地震波速又更高。 The Eurasian Plate subducts to the east beneath the Philippine Sea Plate along the Manila Trench. While the subducting slab is relatively cold and seismically fast as compared to ambient mantle, those of the mantle wedge often exhibit opposite characters resulting from partial melting of wet peridotite. As a result, we expect the existence of lateral velocity variations in slab-normal direction along the Manila trench corresponding to different depths of subducting slab. The objective of this study is thus to test the expectation by analyzing surface wave dispersions as observed by Broadband Array in Taiwan for Seismology (BATS). The fact that earthquake sources distribute on both sides of the Philippine archipelago makes the test feasible with distinct ray coverages to BATS’s stations. Surface wave dispersion refers to the different velocities of waves with different frequencies (periods), which can be explained by the longer period wave sampling deeper structures. In this study, we applied the Multiple Filter Technique (MFT) to analyze the Rayleigh wave dispersion curves. Results thus analyzed are then used for inversion of 1-D velocity model with the AK135 model as the initial, using the damped least-square inversion scheme. We determine the optimal damping factors by grid searching on the inversion results of synthetic waveforms, which are synthesized by FK method. The velocity inversion is done with the conventional surf96 codes. While velocities are overall fast for ray paths to the east of the Manila trench relative to those to the west, explained by the sampling of oceanic lithosphere versus continental one, there are indeed paths of low velocity anomalies corresponding to surface volcanic chains suggesting the effects of magma chambers. Future works include 2-D tomography studies to better resolve the velocity anomalies.
