Site Characteristics of the Puli Area, Taiwan

Abstract

It has been widely documented by now that the intensity of the September 21, 1999 Chi-Chi earthquake resulted in severe damage in the Puli area; regrettably, however, only two strong-motion stations (TCU073 and TCU074) had been installed. To investigate the site characteristics of this area, we carry out dense microtremor measurements at 87 sites. The comparisons of the H/V ratios between the earthquake recordings and microtremor data at TCU074 strongly indicate that with regard to both trend and predominant frequency, they are highly consistent. Undeniably, what this signifies is that the use of microtremors is, in fact, a good alternate for the study of site effects. We also use the GIS to integrate the results of the H/V ratios of the microtremors and the patterns of damage. With the intersection of Chungsan Rd. and Chungcheng Rd. taken as the heart of the town of Puli, we find that the predominant frequencies are at their highest in the center of the town, whereas they are at their lowest in the eastern part. However, we also find relative high predominant frequencies in the range of 2 Hz to 4 Hz in the western part of the town which just happens to be the area which was the most seriously devastated by the Chi-Chi earthquake. As for the amplification factors, based on the contours at different frequencies, at frequencies of 2 - 4 Hz, they are remarkably high in the center of Puli. Besides this, the amplification effects in the northern area where multi-story buildings collapsed are by far the most obvious, with related predominant frequencies between 0.7 Hz and 0.9 Hz. On these grounds, we conclude that the collapse of many structures during the Chi-Chi earthquake was very closely related to the site effects of Puli.

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