NON-PRISMATIC EFFECT ON THE SCATTERING OF SH-WAVES IN 3-D CANYONS

The scattering of seismic waves by topographic irregularities can amplify ground motion and intensify damages such as the activation of landslides and rock failures during earthquakes. The scattering of waves by topographies in canyons causes spatial variation of the earthquake ground motion which should be considered in the analysis and design of large structures located at these sites. It is common for topographic effects of canyons to be analyzed using 2-D models that assume the canyons to be prismatic. In order to investigate non-prismatic effect of 3-D canyon on the scattering of SH-waves two cases are analyzed. First case is the canyon with semi-circular cross section widening toward both ends and the second one is the canyon narrowing toward ends. The analysis uses an extended 3-D multi-domain boundary element method. A comparison of results for prismatic and non-prismatic canyons shows that the non-prismatic canyon could strongly affect the amplification and the variation of ground motion along the canyon. The significance of these effects depends on the pattern of cross-section variation along the canyon length, as well as the incident wave direction. It is shown that non-prismatic effect for the canyon widening toward ends amplifies the topographic effect in respect to the prismatic solution. As arch dam sites resemble this type of canyons, the latter might have a high impact on he seismic loads for the dam. However the canyon narrowing toward ends de-amplify the topographic amplification. Thus it is concluded that for non-prismatic canyons a 3-D solution is essential in order to get an accurate perspective of the phenomena of topography.