1999 集集地震後之黏彈性鬆弛效應;Impact of the Viscoelastic Relaxation Following the 1999 Chi-Chi, Taiwan, Earthquake

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题名:	1999 集集地震後之黏彈性鬆弛效應;Impact of the Viscoelastic Relaxation Following the 1999 Chi-Chi, Taiwan, Earthquake
作者:	蔡宜芬;Yi-Chun Tsai
贡献者:	地球物理研究所
关键词:	黏彈性鬆弛效應;黏滯係數;Viscoelastic Relaxation;viscosity
日期:	2006-07-05
上传时间:	2009-09-22 09:55:49 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	本研究採用中央研究院、內政部以及中央氣象局在台灣地區所設置的GPS連續觀測站資料，以及中央研究院在集集地震後曾實施多次重複觀測的野外測站資料做時間序列分析。我們總共使用64個連續站的資料，時間從2001年1月1日至2005年底，而野外站的部分則是使用103個測站，時間由2000年12月至2005年11月。本研究利用Bernse軟體解算得到每個測站的座標，使用線性回歸的方式來估算野外測站的速度場，而固定站的部分除了修正同震位移量外，還加上年週期及半年週期修正，求出各測站的速度。並利用1992~1999震前已存在之測站的GPS重複觀測資料，以線性內插法求得各測站震前速度，再將各測站速度減去震前速度(間震期速度)，進而推求2001-2005年的5年期間各測站的震後地表位移量，最後和我們的震後黏彈性鬆弛模型結果做比較分析。由觀測資結果顯示，2001-2005年震後五年期間，水平方向及垂直方向震後位移最大值分別為17公分及12公分。為了解下部地殼及上部地函在集集地震後所產生的黏彈性變形，我們使用Sandia National Laboratory所發展的Cubit軟體來建立我們的網格，且利用有限元素法軟體(PYLith)建立模型，並使用適合的黏滯係數於各層，來估算由黏彈性變形所造成的地表變形，進一步與觀測結果比較。利用震後五年的資料，看黏彈性鬆弛效應對於震後變形有多大的貢獻。在測試模型中，我們使用兩層之層狀模型與彈性解析解和黏彈性解析解做比較，發現模型值在彈性解析解中，有良好的擬合，黏彈性半空間解析解中，數值上有些許差異，但因差異很小，仍然不影響其可信度。接著我們使用三層層狀模型與實際觀測值比較，發現在水平方向模型曲線趨勢與觀測值相似，但在數值上卻有很大的差異，進而根據Lin(2000)之結果，在中央山脈下方存在一低黏滯係數之塊體模擬模型。我們改變低黏滯係數的位置、大小、型態、黏滯係數高低來探討這些條件與模型曲線的關係。結果顯示改變位置對於模型曲線的影響最大，當低黏滯係數塊體接觸斷層時，模型曲線會呈現非常大振幅的數值。計算各個模型的Rms值後，發現模型40-60km的垂直低黏滯係數塊體Visc1、及橫向低黏滯係數在40-80km的Visc10其Rms值最小，且將其模型值與實際觀測值做比對發現，Visc1及Visc10在水平方向與觀測值的擬合程度佳，而其他如層狀模型雖然模型曲線趨勢相近，但在值的方面卻有很大的差異，因此我們認為此兩個模型為本研究之最佳解。根據實驗結果可知，在中央山脈底下可能存在一低黏滯係數物質，此物質並未與車籠埔斷層接觸，其型態可能為垂直或水平塊體，大小約為2520km或1040km。但不論我們使用層狀模型或是低黏滯係數塊體模型與實際觀測值做比較，在垂直方向上的擬合度皆不佳，這也許暗示著我們，在震後六年尚有震後滑移(afterslip)影響著地表位移。此外在未來應該尋求更好的三維構造模型，以求在垂直方向上有較佳的擬合度。 We use the GPS data collected from the 64 continuous stations in the Taiwan area, set up and operating by Academia Sinica, Ministry of Interior Affairs, and Central Weather Bureau, and the repeated surveys of 103 campaign sites conducted by Academia Sinica after the Chi-Chi earthquake. The data period is from January 1st, 2001 to the end of 2005. GPS data are processed with the Bernse software V.4.2. To obtain the velocity field of continuous GPS stations, the position time series are corrected for the coseismic offsets and the seasonal periodic motions. We also use the repeated GPS data observed from 1992 to 1999 and apply the linear interpolation to obtain the preseismic velocities for the new sites established after Chi-Chi earthquake. Then the preseismic velocity, is subtracted from the observed velocity for each station to obtain the surface displacement after the earthquake. The maximum horizontal and vertical postseismic displacements for the 5-year period from 2001 to 2005 are 17 cm and 12 cm, respectively. To realize the impact of viscoelastic relaxation on the lower crust and the upper mantle caused by Chi-Chi earthquake, we use the Cubit software by Sandia National Laboratory to establish grids, and the models are constructed by using the finite-element code, PYLith. The appropriate viscosity is adopted for each layer to estimate the surface deformation caused by the viscoelastic relaxation, and further comparisons are made between the model results and the GPS observation. To verify the consistency of the finite-element model with the analytic solution, we start with a two-layer model with a horizontal viscoelastic layer under an elastic layer. In general, the differences between the model values and analytic solution are quite small, indicating the good reliability of finite element models. Afterward we apply a three-layer model to compare with the GPS observation, and we find that the tendency of model curve in the horizontal component is similar to the observations, however, the values are in much difference. Thus we consider to add a block of low viscosity underneath the Central Range following the results of Lin (2000). We change the position, viscosity, and size for the block of low viscosity to examine the relation between the viscosity and the model curve. The results show that the variation of position has the most influence upon the model curve. As the block of low viscosity contacts the fault, the model curve expresses a very large amplitude. We find that model with the vertical low viscosity block located at 40-60km from the fault (Visc1), and another model with the horizontal low viscosity block located at 40-80km from the fault (Visc10) have the minimum RMS values. Moreover, comparing the model curves with GPS data, we find that models Visc1 and Visc10 are more or less consistent with the observation. Accordingly, we consider these two models are the best solutions in this research. We inferred that a low viscosity material may exist beneath the Central Range, and it does not contact with the Chelungpu fault. However, the best models are still not fitting the vertical data well. A more sophisticated three-dimensional model is needed in the future studies.
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