Canyon topography effects on ground motion at Feitsui damsite

The free-field accelerograms along Feitsui Canyon are analyzed and modeled by a numerical scheme to study the effect of canyon topography. Since six strong-motion accelerometers (SC1-SC6) were deployed along the Feitsui Canyon in 1991; there are 14 earthquakes (4.9 less than or equal to M-L less than or equal to 6.6) recorded by these stations until June 1996, but only five triggered all six stations. The maximum PGA value is 68.6 cm s(-2) recorded at station SC1. According to the present data, the effect of the dam on the ground motions at canyon stations can be negligible. The amplitude of ground motion on the slopes of the canyon is bigger than that at its trough. The integral equation method is applied to a two dimensional model of Feitsui Canyon to study the effects of the canyon topography. We choose the ground motion of SC3 or SC4 station at the trough of the canyon as the input motion for the model, which is then used to predict the ground motion at the other five stations. Apart from the earthquake close to the damsite, the simple model can reproduce the observed accelerations at all frequencies below 4 Hz. Overall, the numerical method can well predict the ground motion along the canyon, although the high-frequency simulation is underestimated. (C) 1998 Elsevier Science Ltd. All rights reserved.