Site Effect Analysis From the Records of the Wuku Downhole Array


The first downhole array in the Taipei Basin was installed in the Sewage Disposal Plant of the Wuku Industrial Area at the end of 1993. It is a subproject o ¡§An Integrated Survey of the Sbusurface Geology and Engineering Environment of the Taipei Basin¡¨ which has been conducted since 1991 by the Central Geological Survey, Ministry of Economic Affairs. The main purpose of this downhole array project has been to study the basin effects on seismic waves. By the end of November 1994, 16 earthquakes were recorded, some analysis can already be done. Through the travel time analysis of seismic waves, the results of Vp and Vs for the soil layer from surface to the depth of 352 meters is calculated. On the basis of a comparison of the peak ground motions at different depths with that at ground surface, it is determined that the main amplification exists in the top 60 meters of the soft soil layer. No nonlinear amplification effect occurs in this weak motion data set. The equation representing the variation of horizontal peak groubd acceleration with respect to depth is obtained by using regression analysis. Finally, the ground responses are analyzed in the frequency domain. The spectral ratios between station pairs of different depths show variations of amplitude at different frequencies, and they verity the average velocity between each station pair.

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