A Case History on Soil and Topographic Effects in the 7th September 1999 Athens Earthquake

Large concentration of heavy damage to residential and industrial buildings occurred in the small community of Adámes, near the banks of the Kifisos river canyon, during the 7-September- 1999 Earthquake. To explore whether the particular topographic relief and/or the actual soil profile have 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. Soil layering and stiffnesses are determined from 10 SPT-boreholes and 4 crosshole tests. Ricker wavelets and six realistic accelerograms are used as excitation. The results show that the 2D topography effects are substantial only within 50 meters from the canyon ridge, but 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 (defined as the 2-D / 1-D Fourier spectral ratio) varies from 1.5 to 2.0 over a broad frequency band which covers the significant excitation frequencies. At the location of four collapsed buildings, about 250 in from the edge, 2D (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.