| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 102 === 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.
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