| Summary: | 碩士 === 國立中央大學 === 地球物理研究所 === 99 === 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.
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