Published by Elsevier Science Ltd. All rights reserved 12 th European Conference on Earthquake Engineering Paper Reference: 844 “COMPLEX” SITE EFFECTS IN EARTHQUAKE GROUND MOTION, INCLUDING TOPOGRAPHY E. Faccioli, M. Vanini, L. Frassine Politecnico di Milano, Department of Structural Engineering, Milan, Italy ABSTRACT In the first part, evidence of complex site effects in 2D geological configurations is illustrated, and the related difficulties in interpreting ground motion observations is discussed, using weak motion data recorded in three Alpine valleys in Northern Italy. While, on one hand, such valleys are mostly located in regions of low or moderate seismic hazard, where ground motion data are scarce, on the other hand investment in buildings and infrastructure has been substantial and may not always have been protected by adequate earthquake resistant provisions and zoning criteria. Moreover, experience and data obtained in regions hit by destructive earthquakes cannot be easily transferred to such a different context. The combined influence of 1D propagation effects on the dominant frequencies of motion, and of 2D effects on the amplification level and significant frequency band, are highlighted. For what concerns the influence of the surface relief, the incidence of topographic amplification is investigated (in the perspective of ground motion scenarios) for one historical earthquakes in a mountainous region, i.e. the destructive 1887 Western Liguria (Italy) earthquake which represents a relevant case in point. KEYWORDS Earthquakes, site effects, Alpine Valleys, soil amplification, topographic amplification, weak motion, 2D numerical simulations, ground motion scenarios. INTRODUCTION It is well established that local soil conditions and, to a more limited extent, irregular surface topography can exert a crucial influence on the severity of building damage and its spatial distribution during earthquakes. Engineers have traditionally evaluated such influence using simple models based on a 1D description of local soil profile and seismic wave propagation, with reasonable success. However, recent events such as the 1995 Hyogoken Nanbu (Japan) earthquake with its narrow “intensified damage” belt crossing the city of Kobe and causing the death of over 6000 people, have disclosed a remarkable complexity in seismic amplification patterns due to unfavourable combinations of seismic source and near-surface geology.