| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 103 === The resolution and accuracy of the crustal models obtained from seismic tomography is very important in making reliable estimations of strong ground motions of earthquakes for both post-earthquake disaster relief and assessing future seismic risk purposes. Drastic topography changes have significant effects on the propagation of seismic waves, which leads to perturbations in waveforms and therefore the arrival times and amplitudes of seismic phases. Various factors contribute to the topography-induced waveform changes, including not only the fixed geometrical shape of the surface itself, but also the path-related incidence angles and azimuths of the seismic waves which are dependent on the location of the receiver site as well as the back azimuth, epicentral distance and depth of the earthquake. In order to assess the potential bias in crustal models obtained by traveltime tomography when the effect of surface topography is neglected, we carry out a systematic analysis of the surface topography effect on the traveltimes and amplitudes of seismic waves using accurate synthetics calculated by the finite-difference method in three-dimensional structure with realistic surface topography and quantify the topography-induced delay times and amplitude anomalies by cross-correlations of waveforms obtained with realistic topography and those with topography ignored. The quantitative measurements of topography effect is used to assess the potential biases in seismic tomography due to the omission of topography in modeling regional seismic waveforms. The results show that for the topography relief of northern Taiwan, these biases can be up to 0.4 s for P- and 0.6 s for S-wave delay times and more than 100% for S-wave amplitudes, which we demonstrate to be significant in tomography inversions for crustal structure. Our results also suggest that in PGV and PGA predictions for hazard purposes, results from a few scenario earthquakes are insufficient in producing the complete picture.
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