The ground response and site effects of a steep valley were verified with a two dimensional plane strain analysis in the time domain against recorded motions.Weak motions (<0.15 g) have been measured on the site with three accelerographs placed at the base and crests of the slopes. The instrument at the base is on rock and the ones towards the crests on soil. Dynamic models were done with the finite element method (FEM), loading at the base the signals recorded on rock. The peak ground accelerations (PGA) obtained with the FEMfit with the ones measured in situ. The comparison between the main frequencies of the events and the fundamental frequencies of the site show resonance effects. The raised amplifications were demonstrated to be caused by geological site effects, related to the upper geotechnical unit and not by the topography. These results show the importance of subsurface structure in causing resonance effects.
Numerical methods have been recently applied for back and parametric analysis of earthquake ground response and site effects (e.g.Athanasopoulos et al., 1999; Havenith et al., 2002; Lokmer et al., 2002; Paolucci, 2002; Papalou & Bielak, 2004; Bouckovalas & Papadimitriou, 2005; Psarropoulos et al., 2007). There are few attempts to verify measured with modelled data in time and frequency domains accounting for topographic and geologic effects separately (e.g. Semblat et al., 2005; Assimaki et al., 2005). Thimus et al. (2006) present a theoretical verification of wave propagation with finite differences (FDM). Sincraian & Oliveira (2001) had field measurements but could not find a good fit with the 2− and 3D FEMthey used. Geli et al. (1988) found that 2Dmodels underestimate field observations, caused by the simplicity of the models. Only the 3D model in frequency domain showed a good match. They concluded that for peak ground accelerations (PGA) smaller than 0.24 g hills behave approximately linear. Users of these tools should be aware of the limits of their applicability and benchmark results.Weak motions have been validated with field data with a 3Dhybrid approach (indirect boundary elements method) by LeBrun et al. (1999). They found a good correlation on the responses for frequencies lower than 1 Hz. On the other hand, 1 − Danalysis methods have been extensively verified as documented by Kramer & Stewart (2004) with nearby rock-soil signals aswell as vertical arrays. Strong amplifications have been measured in some cases on hills (Bouchon & Barker, 1996) and related to topographical site effects, without quantifying the effects of the local geology. This paper presents the comparison of the ground responses modelled with two-dimensional (2D) linear elastic finite elements (FEM) of a site with field measurements, comparing the site effects of topography and geology. The site is located in a highly seismic region of Costa Rica.
