Non-Prismatic Effects of Three- Dimensional Canyon Sites on the Diffraction of Seismic Waves

The diffraction of seismic waves by topographic irregularities can dramatically amplify free field ground motion. This phenomenon was experienced and observed in figure of local damage concentration, landslides and rock failures during different past earthquakes. It is common for topographic effects such as canyons to be analyzed using 2-D models that assume the canyons to be prismatic (i.e., have uniform cross sections). The present paper investigates the effect of a non-prismatic 3-D canyon on the diffraction of seismic waves. The analysis uses 3-D boundary element method. A comparison of results for prismatic and non-prismatic canyons shows that the non-prismatic canyon can increase or decrease the amplification and the variation of ground motion across 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 and frequency. For the most common cases in practice (the non-prismatic canyon widening toward the ends) this effect amplifies the topographic amplification strongly. 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.