| dc.description.abstract | Central Taiwan suffered very severely damages during the Chi-Chi earthquake in 1999. It includes buildings collapsed, casualties, and economic loss, etc, and the Chelungpu fault ruptured during this earthquake. Although the ground motions in the footwall area were smaller than that in the hanging wall area. But the Taichung area which located in the footwall also has many building damages occurred. In order to obtain more information and understand the local site effect during earthquake, we performed the dense microtremor measurement in the Central Taiwan area. In this study, we set about 500 microtremor stations. By using the horizontal to vertical spectral ratio（H/V Ratio）, we can analysis the site response in the central Taiwan. The site response can be used to compare with the ground motion response and the damage distributions of the Chi-Chi earthquake.
The dominant frequency and amplification factor at each station were calculated. From the dominant frequency contour map shows that at the north of Taichung basin west of Houli tableland, Tatu tableland, and north of Pakuashan tableland, the dominant frequency are about 8 ~ 16 Hz; the western foothill zone is about 3.5 ~ 7 Hz; the south of Taichung basin is about 2 ~ 5 Hz; Chingshui and Changhua coastal plain are about 0.5 ~ 2.5 Hz. We also calculated the “Kg value” (Nakamura, 1996) for estimating the earthquake damage of ground liquefaction area. In this study, when Kg value great than 20, it had good correlation with liquefaction area during Chi-Chi earthquake. So the Kg value of microtremor offers a good alternative for estimation of an area’s potential for liquefaction.
For the purpose of the earthquake resistant design, earthquake engineers must consider the site response at a specific period. Therefore, in this study, we select 4 frequencies (5.0, 2.0, 1.0, 0.6 Hz) to plot out the amplification contour map of the site response and compare with the distributions of damaging buildings during the Chi-Chi earthquake. The results show the site effect caused many building damages in foothill area during the Chi-Chi earthquake in 1999 except the liquefaction areas. Otherwise, the damage buildings’ floor higher than 10 may resulted by the stress propagation because the Chelungpu fault trace turned to right nearby the Wufong area, but we need more study in the future. On the basis of this analysis, the results can also use as a reference for the future seismic hazard mitigation, seismic safety design, land use planning, and estimate potential of soil liquefaction. | en_US |