利用三維有限差分法模擬與分析台北盆地的場址放大效應

3D Finite Difference Simulation-Based Site Response Analysis of Taipei Basin

王櫻儒(Ying-Ru Wang)

指導教授 ： 陳浩維

國立中央大學/地球科學學院/地球物理研究所/碩士(2007年)

摘要

在最近幾次大地震中，如921或331地震，雖然震央位置不在台北盆地附近，但盆地內受不等程度的搖晃與建物破壞影響，有時卻比處在震央附近的地區還要來的大。這是由於地震波傳經盆地內部時，大部分的能量會因盆地形貌、地形與低速區而被侷限在盆地內，其中某些特徵頻率的震波會因產生共振而造成場址放大效應，進而危害建築物。本篇文章利用三維有限差分法模擬震波到達台北盆地時的傳遞情形，並且使用單站頻譜法，對中央氣象局提供的四獸山地震強地動資料以及合成震波圖進行處理，分析影響盆地及其周邊區域的盆地放大效應之因素。
由三維模擬結果可得知，盆地放大現象主要受到盆地形貌、沖積層厚度變化以及松山層速度特徵等影響。由於本研究使用近震源的四獸山地震以及台北盆地附近的強地動測站，且因模擬模型之自由表面最高高程只約為海拔10-15公尺，所以本研究不考慮地表地形變化。計算三維全波場的模擬方法自動包括了一維以及二維反應的影響。
實際資料之共振主頻為低頻(1.0Hz)，但合成資料之共振主頻值卻較高(2.0Hz)，造成兩者間些許差異之主因，是由於模擬模型沒有考慮到更精確的三維速度模型、更深層地層對於地震波傳遞時之折射/反射/散射等現象、以及Q值對震波之影響等。次要原因則是由於實際資料為寬頻資料，而合成資料較為窄頻，若將不同控制主頻之合成資料相加再進行資料分析，可發現到合成資料之共振主頻值較偏低頻(0.5Hz)，且接近實際資料之主頻值。使用不同的控制主頻進行三維數值模擬，但利用HVSR分析所得的共主頻值卻是相似的，皆指出模擬模型的主要共振頻率值為0.5/1.0及2.0/2.5Hz，且此2.0Hz的頻率與利用40米沖積層計算之理論共振頻率值相符合。變更震源位置(輻射型式)，其HVSR結果會跟著改變；而淺源地震所造成的地震紀錄以及HVSR會比深源地震來的複雜。
由震波模擬與頻譜法分析結果可知，盆地邊緣(即沉積層與基盤介面)，淺部三維速度的微區分、非線性波場模擬、更精準的盆地三維型貌及松山層與基盤間地層層序跟速度分佈的進一步訂定和勘探，仍屬未來相關研究的必要目標。

關鍵字

場址效應 ； 有限差分法 ； 頻譜比 ； 台北盆地

並列摘要

Serious seismic disasters may highly relate to basin amplification effects caused by the shallow earthquake or by the passage of seismic waves with rather large strong ground motion generated from large earthquake occurred distance away. Although the epicenters of these damage earthquakes are far away from the basin, the intensity within the basin is even larger than the area near the epicenter. Such amplification of seismic energy within the basin region is well-known because most seismic waves can be trapped by the basin geometry, topography and the extremely low velocity layer sitting on top of Taipei basin. Among all the previous studies, we provide relatively new approach by using the 3D hybrid discontinuous-grid finite difference computational method developed in-house to simulate the seismic waves propagating within the Taipei basin. In addition, we also implement the method of horizontal-to-vertical spectral ratio, which has been widely used by the engineering seismology, to analyze site amplification in and around Taipei basin. The main goal is to investigate the factors affecting seismic resonant frequencies (SRF) and spatial variation of amplification from 3D effects. The HVSR analysis is applied to both synthetic and Sishou Hills earthquake data collected from TSMIP of Central Weather Bureau, Taiwan.
From our 3D simulation studies, basin amplification and HVSR results are mainly affected by the basin shape, spatial variation of sedimentary layer thickness and the possession of low velocity feature carrying by the Songshan formation. Topography effect is not included in our 3D computation as strong motion records collected from Sishou Hills earthquake is mainly from stations located within Taipei basin where rather flat free-surface with elevation high of no more than ten to fifteen meter above sea surface. Full 3D waveform simulation approach automatically includes effects from 1D and 2D responses.
Over all speaking, the results from HVSR analysis of real data has lower SRF of 1.0Hz while the synthetic one poses a dominant feature of having higher SRF of 2.0Hz. Such difference is mainly attributed to the insufficient model specification on Q factors, more accurate and high resolution 3D reference velocity model of Taipei basin and lacking of highly correlated reflection, refraction and scattering signals from deeper layers in our 3D simulation. An added factor is that synthetic data is fairly narrow band in contrast to real one which has relatively wide frequency contents. Sum over all synthetic frequency responses improve the low frequency contents and more reasonably fits the real data records. Comparing HVSR analysis results from different frequency band, the resonance frequency around 0.5, 1.0, 2.0 and/or 2.5 Hz do exist in all calculations. The 2.0Hz peak value may well corresponds to the southeast (SE) part of Taipei basin where Songsan formation becomes shallower. Such response mainly corresponds to the 40 m layer thickness exist in the SE of Taipei basin where relatively simplified 1D layer model produces main wave modes due to the resonance of waves existing within the region. Providing that source radiation may slightly affect the HVSR analysis, the investigation on fundamental resonant frequency is still feasible. Shallow earthquake produces more complicate waveform records compare with simulated records from deeper one.
The results from 3D seismic simulation and site response analysis show that further detailed investigation on basin edge, lateral variation of 3D shallow velocity in the Songsan Fm., accurate shape of basin basement, more detailed velocity distribution for the deeper layers and nonlinear waveform simulation scheme are the imperative research targets in the future.

並列關鍵字

Taipei basin ； finite difference ； spectral ratio ； site effect

參考文獻

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