||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.
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