Strong Ground Motion Characteristics of the Chi-Chi, Taiwan Earthquake of September 21, 1999

The Chi-Chi, Taiwan earthquake produced a rich set of 422 strong ground motion recordings. In this paper we present some results from our analysis of these recordings. First, we found that the overall level of the observed horizontal peak ground acceleration (PGA) values was relatively low (about 50% less) when compared with what would be predicted for an earthquake of the same magnitude by existing attenuation models based on worldwide data. High horizontal PGA values at sites on the hanging wall and within 20km of the surface fault ruptures are notable exceptions. The horizontal PGA values are indistinguishable among the four different site classes. However, the horizontal PGA values in Taipei Basin, Ilan Plain, and Hwalien area are significantly higher than the average at similar distances. The vertical PGA values on the average are about 0.6 times their horizontal counterparts. Unlike the horizontal PGA, the observed horizontal peak ground velocity (PGV) values are relatively high (about 80% higher) when compared with what would be predicted for an earthquake of the same magnitude by an existing PGV attenuation model based on worldwide data. In summary, as far as peak ground motion parameters are concerned, the Chi-Chi earthquake may be called a high-PGV, low-PGA (HV-LA) earthquake. Next, we analyzed the 5% damped acceleration response spectrum shapes for the four different site classes B, C, D, and E, in order to study possible dependence of the response spectrum shape on local geologic site conditions. It is found that the peak spectral amplification factor ranges between 2.3 and 2.5 for all four classes of site conditions. In general, the response spectrum shape for Class B sites on soft rocks older than the Pliocene age has spectral amplification for periods up to about 1.5 seconds. The spectral amplification of Classes C and D sites on stiff soils occurs over periods up to about 2.0 seconds. The spectral amplification of Class E sites on soft soils occurs over periods up to about 3.0 seconds. The response spectrum shapes for Taipei Basin and Ilan Plain are quite similar to Class E sites, whereas Hwalien area is similar to Class C Earthquake Engineering and Engineering Seismology 1 Volume 2, Number 1, March 2000, pp. 1–21 2 Earthquake Engineering and Engineering Seismology, Vol. 2, No. 1 or D sites. Finally, we analyzed the observed characteristics of acceleration response spectra from 44 near-fault sites. For the eight sites within 2km from the surface fault ruptures, the median horizontal PGA value is about 0.5g. The corresponding spectral peak is about 1.0g. The median-plus-onestandard-deviation horizontal PGA value is about 0.7g and the corresponding spectral peak is about 1.8g. Thus, for sites within 2km from the surface fault ruptures, application of a scaling factor of 1.5 to the current seismic design spectrum anchored at a PGA value of 0.33g for Zone 1A appears to be appropriate. For the 18 sites located at 2 to 10km from the surface fault ruptures, the median horizontal PGA value is about 0.25g. The corresponding peak spectral value is about 0.6g. The median-plus-one-standard-deviation horizontal PGA value is about 0.4g and the corresponding peak spectral value is about 0.8g. Thus, for sites between 2 and 10km from the surface fault ruptures, the current seismic design spectrum anchored at a PGA value of 0.33g for Zone 1A appears to be adequate. For the 33 sites at 10 to 20km from the surface fault ruptures, the median horizontal PGA value is about 0.18g and the corresponding peak spectral value is about 0.45g. The median-plusone-standard-deviation horizontal PGA value is about 0.3g and the corresponding peak spectral value is about 0.7g. Thus, for sites located between 10 and 20km from the surface fault ruptures, the current seismic design spectrum anchored at a PGA value of 0.33g for Zone 1A is more than adequate.