Topography and Soil Effects in the M S 5.9 Parnitha (Athens) Earthquake: The Case of Adámes

Large concentration of damage to residential and industrial buildings occurred in regions near the banks of the Kifisos river canyon during the 7-September-1999 Parnitha (Athens) Earthquake. One such region, which experienced unexpectedly heavy damage, was the small community of Adámes, which borders the canyon near its deepest point. To explore whether in addition to structural factors the particular topographic relief and/or the actual soil profile contributed to the observed concentration and non-uniform distribution of damage within a 300 m zone from the edge of the canyon cliff, wave propagation analyses are conducted in one and two dimensions. Finite-element and spectral-element formulations are used to this end. To avoid spurious wave reflections at the artificial boundaries, ourtwo-dimensional (2-D) finite-element analyses utilize Bielak's Effective Seismic Excitation method. Soil layering and stiffnesses are determined from 10 SPT-boreholes and 4 crosshole tests. Ricker wavelets and six realistic accelerograms are used as excitation; two of the latter are selected from the literature and four are obtained on the basis of the four strongest motions of the earthquake, recorded in central Athens. The results show that the 2-D topography effects are substantial only within 50 meters from the canyon ridge. These effects materialize only in the presence of the relatively soft soil layers that exist in the profile at a shallow depth. The so-called Topographic Aggravation Factor (TAF), defined as the 2-D over 1-D Fourier spectral ratio, varies around 1.4 over a broad frequency band which covers the significant excitation frequencies. At the location of four collapsed buildings, about 250 m from the edge, 2-D (topography) effects are negligible, but the specific soil profiles amplify one-dimensionally all six ground base excitations to spectral acceleration levels that correlate well with the observed intensity of damage, at least in a qualitative sense. Copyright Kluwer Academic Publishers 2002