Numerical modelling of seismic slope stability

Earthquake ground-motions recorded worldwide have shown that many morphological
and geological structures (topography, sedimentary basin) are prone to amplify the
seismic shaking (San Fernando, 1971 [Davis and West 1973] Irpinia, 1980 [Del Pezzo et al.
1983]). This phenomenon, called site effects, was again recently observed in El Salvador
when, on the 13th of January 2001, the country was struck by a M=7.6 earthquake. Indeed,
while horizontal accelerations on a rock site at Berlin, 80 km from the epicentre, did not
exceed 0.23 g, they reached 0.6 g at Armenia, 110 km from the epicentre. Armenia is located
on a small hill underlaid by a few meters thick pyroclastic deposits. Both the local
topography and the presence of surface layers are likely to have caused the observed amplification
effects, which are supposed to have contributed to the triggering of some of the
hundreds of landslides related to this seismic event (Murphy et al. 2002). In order to better
characterize the way site effects may influence the triggering of landslides along slopes, 2D
numerical elastic and elasto-plastic models were developed. Various geometrical, geological
and seismic conditions were analysed and the dynamic behaviour of the slope under
these conditions was studied in terms of creation and location of a sliding surface. Preliminary
results suggest that the size of modelled slope failures is dependent on site effects.