Soil-Dependent Topographic Effects: A Case Study from the 1999 Athens Earthquake

Abstract

In theMs5.9 Athens, Greece, earthquake, surprisingly heavy damage occurred on the eastern bank of the Kifissos River canyon. To explore whether the particular topographic relief and/or the local soil conditions have contributed to the observed concentration and non-uniform damage distribution within a 300-m zone from the canyon crest, we conduct finite-element analyses in one and two dimensions, using Ricker wavelets and six realistic accelerograms as excitation. The nonlinear soil response is simulated in the time-domain using a hyperbolic stress-strain model, and also approximated using a modified equivalent-linear algorithm; results obtained by means of the two methods are discussed in detail. Our simulations show that topographic effects are substantial only within about 50 m from the canyon ridge, materializing primarily because of the presence of relatively soft soil layers near the surface of the profile. We then introduce the concept of two-dimensional/one-dimensional response spectral ratio to describe the effects of topography as a function of local soil conditions, and suggest a frequency- and location-dependent topographic aggravation factor to be introduced for the modification of design spectra in a seismic code.