| 摘要: | 場址效應於強地動研究中扮演著關鍵的角色，當地震波由堅硬的岩盤傳遞至鬆軟的沈積物時，震幅將產生放大的作用，大大增加地震造成的危害。在工程地震學中，地表下30m平均剪力波速(VS30)被廣泛用來作為評估場址效應的參數，可以根據孔隙比以及垂直有效應力，配合剪力波速轉換公式對大於30m深的資料進行估算，當資料深度小於30m時，可以藉由外插法進行推估。本研究蒐集並檢核強震站資料庫(EGDT)中的物性試驗資料以及波速量測資料，並根據2000年至2008年的資料庫，提出孔隙比、垂直有效應力對剪力波速之轉換公式。繼之，利用剪力波速轉換公式以及剪力波速外插法，對中央地調所(CGS)的鑽孔進行各深度之Vs推估，並求得VS30。最後，利用Kriging with varying local means方法繪製台北盆地VS30分布圖。
結果而言，台北盆地VS30的分布範圍多落於157(m/s)至640(m/s)，根據此一分布，在台北盆地有三個區域（北區、東北區、西南區）的VS30小於210(m/s)；盆地兩側之南緣則大於300(m/s)；盆地中心則落於210至300(m/s)之間。本研究在VS30小於210(m/s)之區域，相比郭俊良(2021)之結果有更好之成果，顯示在納入更多資料後，改善了VS30的推估結果。此外，在台北盆地北、東北、以及盆地中心之區域，本研究相比於過去研究有較低之殘差值。
關鍵字：VS30、孔隙比、有效應力、台北盆地、場址效應
;Site effect plays an important role in issues of strong ground motion studies. Site effect is caused by soft deposits overlaying hard rock, causing seismic ground motions to be amplified and increasing damage during large earthquake. The time-averaged shear-wave velocity in the upper 30 meters, i.e. VS30, is the most popular and widely be used site parameters representing the site effect in engineering seismology. In this case, VS30 can be estimated based on void ratio and effective vertical stress by transformation functions with data within 30 meters. An extrapolation method:Conditional Independent Property (CIP) model by (Dai et al., 2013) is applied when available data is less than 30 meters. The data of soil physical property data and wave velocity measurements from the Engineering Geological Database for TSMIP (EGDT) were collected and checked, we then proposed transformation functions to predict the shear-wave velocity (VS) using void ratio and effective vertical stress based on the database from 2000 to 2008. Using both transformation functions and extrapolation method, the VS at each depth from numerous drilling boreholes from Central Geological Survey (CGS) has been estimated to obtain VS30 for each borehole. Finally, Kinging with varying local means is used to create a distribution map of VS30 in the Taipei Basin.
Based on regression analysis for sand, silt and clay in the Taipei Basin with EGDT data from 2000 to 2008, resulting in improved RMSE values compared with (Kuo, 2021) have results such as Transformation function for Sand: RMSE from 45.8 m/s decrease of 30.8 m/s and transformation function for Silt and Clay: RMSE from 47.9 m/s decrease to 29.2 m/s. In addition, by separating clay and silt, we have two transformation function better than combined with RMSE 26.52 m/s and 29.86 m/s, So three transformation functions estimate VS30 for CGS boreholes: Sand, Silt, Clay.
The number of boreholes for (Kuo, 2021) result is about 1914 boreholes, and this study updated to 5773 boreholes, include equal to or larger than 30m and less than 30 meters for Kriging with varying local means.
Overall, according to the study, the VS30 distribution in the Taipei Basin ranges from approximately 157 m/s to 640 m/s. Based on this distribution, three specific areas can be identified:North, Northeast, and Southwest Areas: These areas have VS30 values less than 210 m/s.South to East Margin of the Taipei Basin: This area has VS30 values larger than 300 m/s. Center of the Taipei Basin: The VS30 values in this area range from 210 to 300 m/s.
This study found that areas with VS30 values less than 210 m/s in the Taipei Basin exhibit higher accuracy compared to (Kuo, 2021) results and suggests that incorporating additional data in these regions has improved the prediction of VS30 value. In addition, in North, Northeast and Center of Taipei Basin, this research provides less residuals values of VS30 comparing with (Kuo, 2021)
Keywords: VS30, void ratio, effective stress, Taipei Basin, site effect, CIP model |
