Abstract
In this paper, a total of 901 free-field accelerograms recorded at the stations in west-central Taiwan from 32 earthquakes that occurred in Taiwan area between 1993 and 1999 are separated as four data groups based on PGA levels, the station sites are classified into three types by their dominant periods of acceleration response spectra. The characteristics of response spectra in regard to both earthquake intensity and station type are analyzed. In order to highlight the behavior of the Chi-Chi mainshock, several kinds of comparisons of the near-fault responses have been made with those of far-field data. The results show that the response spectra increase generally with the earthquake intensity for all stations. The spectral shape in long period regions become more predominant if the soil condition is softer. The mean response spectra plus one standard-deviation of the Chi-Chi mainshock are significantly larger than the required values of the current seismic design code. In terms of the long-period range, the spectra of the Chi-Chi mainshock are larger than the mean value of 32 events by more than 200 times. The spectral ratios of vertical to horizontal component almost exceed 2/3; this phenomenon reflects the intensified vertical ground motion due to the thrusting movement of the Chelungpu fault. Therefore, a modified design code is suggested for applications to near-fault areas.
Highlights
The importance of the response spectra approach for structural design is well known to earthquake engineers and seismologists
The strong ground motion data recorded by the network of Taiwan Strong Motion In strumentation Program (TSMIP) of the Central Weather Bureau (CWB) are used to analyze the characteristics of response spectra
The peak spectral amplification factors obtained in this study, 2.3, 2.4, and 2.2 for site types A, B, and C, respectively, are smaller than those given by Seed et al (1976), 2.6, 2.9, 2.8, and 2.4, suggesting that the site classification in this study is not appropriate, or the curves given by Seed et al(1976) may not be useful for Taiwan areas
Summary
The importance of the response spectra approach for structural design is well known to earthquake engineers and seismologists. Tsai and Huang (2000) analyzed the acceleration response spectra near the Chelungpu fault by data of the Chi-Chi earthquake, and show that the median of spectra accel eration will increase as the distance from the recording site to the rupture surface decreases. On September 21, 1999, a disastrous earthquake (the Chi-Chi mainshock) of magnitude ML;;::;.7.3 struck this area (Wang et al 2000). The station sites are classified as three types by their dominant periods of acceleration response spectra. The characteristics of response spectra in regard to both earthquake intensity and station type are analyzed. Both the appropri ateness of site classification and the problems of structural seismic design will be discussed. In order to highlight the behavior of the Chi-Chi mainshock, several kinds of comparison on the near-fault responses are made with those of far-field data
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