鯉魚潭水庫位於台灣中部地區，壩高97公尺，壩身設有七個地震儀。本研究採用了1999年到2002年壩體完整地震紀錄共50筆，其中包括了1999年的集集地震，藉以瞭解壩體受震時的振動特性。利用壩頂和壩底測站的傅氏譜與反應譜比來確定壩體受震時的振動頻率，可以發現其顯著的振動頻率會隨著壩體受震強度增加而減少。 垂直向的顯著頻率在小規模地震下大致上都集中在3Hz，而集集地震時其頻率則集中在2Hz。垂直壩軸與平行壩軸方則約略集中2Hz~1.27 Hz與2Hz~1.51Hz(小規模地震到集集地震)附近。壩頂與壩底測站的PGA比值幾乎都大於1，且其值會隨著壩底受震強度的增加而減小。統計所有地震記錄後可以發現在垂直壩軸與平行壩軸兩方向其值大約在1.7~4，而垂直向的比值則集中在1.4~3。 最後採用黃鍔等人在1998年發表的經驗模態分解法，將壩頂與壩底測站的加速度歷時曲線分解成多個由高頻到低頻的內建模態函數。比較壩頂與壩底測站的內建模態函數，可以發現壩體受震時其低頻的部份兩者幾乎重合，而在高頻的部份則有放大的現象。 Seven seismographic stations were installed at the Liyutan dam, a zoned compacted earth dam with a height of 97m, which is located in central Taiwan. During the period of 1999-2002, a large amount of seismic motions were successfully recorded for about fifty earthquake events which included the 1999 Chi-Chi earthquake. The author used these records to analyze the vibration characteristics of the Liyutan dam. Fourier spectrum and response spectrum ratios between the records at the top and bottom of the dam were used to identify the fundamental frequencies of the earth dam. It clearly shows a gradual decrease of the vibration frequency from small to large excitation level. The vertical vibration frequency of the earth dam ranges from about 3 Hz under small excitation (several gals) to about 2.0 Hz under large excitation (Chi-Chi earthquake with a base PGA level of 105 gal). For motions in directions transverse and parallel to dam axis, the fundamental vibration frequencies of the dam are from 2 Hz to 1.27 Hz, and from 2 Hz to 1.51 Hz. The PGA ratios between the records at the top and bottom of the dam are generally greater than 1.0, which means amplification, and decrease with the increase of the base excitation level. On the average, the PGA ratios in directions transverse and parallel to dam axis range from about 4 under small excitation (several gals) to about 1.7 under large excitation (Chi-Chi earthquake with a base PGA level of 128-144gal), however, the PGA ratios in vertical direction only range from about 3 to about 1.4. An empirical decomposition method proposed by Huang et al. (1998) is used to decompose the motion histories at the top and bottom of the dam into several intrinsic mode functions (IMFs) which are orthogonal and complete. By comparing these IMFs, we found that the low frequency parts of seismic motions at the top and bottom of the dam are nearly the same, however, the high frequency part is significantly amplified when the motion propagates from the bottom to the top of the dam.
