| Summary: | 碩士 === 國立中央大學 === 地球物理研究所 === 95 === Located in the Northern Taiwan, Taipei is the city where dense population and many high rise buildings like “Taipei 101” may cause concerns on earthquake hazards. Large amplification effects may cause serious damages when seismic waves impinging upon soft soil layer sitting on the top of Taipei basin. Three typical earthquake data sets including Chi-Chi earthquake (Sept. 21., 1999); Hualien earthquake (March 31, 2002) and Shisoshan earthquake (Oct. 23, 2004) were used for site amplification effects studies. Based upon spectral ratio analysis technique for S-wave and noise wave phases, the dominant resonant frequencies ranging 0.5 to 2.0 Hz was defined in the Taipei basin. Higher resonance frequency can be obtained in area with obvious topography variations. The estimated amplification values from spectral analysis base on HVSR methods appears to have higher response than HVNR method. Base on HVNR study, all three earthquakes data show strong amplifications and higher resonant frequency from topography around Taipei basin. More resolvable resonance frequency can be obtained from know S-wave than noise phases. Directional spatial distribution of amplification is mainly correlated to the types of wave phases used in HV method. For Sv-wave, direction of propagation is roughly perpendicular to the trend corresponds to high amplification area. While base on noise data, mainly corresponds to Raleigh waves, the direction of propagation is parallel to the trend with high amplification values. The large magnitude including 921 and 331 earthquakes will have less site effects resulting from topography than basin amplification effects. For smaller Shisoshan earthquake, a topography effect becomes more apparent. Combination of path and site effects produces even more complicated resonant frequency and amplification patterns.
Base on the calculations of reflection and transmission coefficients varies with incidence angle, drastic increases in amplitude as conditions meets the total reflection. From a simple model, we have found positive relationship between fundamental resonant frequency and S-wave speed but inverse proportional to the four times of layer thickness. Incidence angle, S-wave velocity, density and quality factor are the main parameters affecting amplification and Horizontal to vertical signal spectral ratio. The changes in impedance contrast may not strongly affects the resonant frequencies and conclusions obtained from high contrast model. Two dimensional SH wave simulation of basin structure shows that: the shallow S-waves velocity specification is critical for the quantitative analysis of basin amplification effects. With increasing details in ultra-shallow S- wave velocity variations, the estimated cumulative kinetic energy from synthetic data increases accordingly which also indicates that thicker layer will preserve more kinetic energy obtained from recorded strong motion data. Basin edges also show larger kinetic energy than basin center which explain well with the observed phenomena. For further detailed simulation, a synthesis of plane waves impinging upon formation boundary with different incidence angles for basin amplification effects studies should be actively pursuit. From our preliminary simulation, results indicate that it is important to systematically measure and set up a near-surface S-wave velocity database in Taipei metropolitan area for future more practical usages in strong motion analysis and interpretations.
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