Complex seismic site effects in sediment-filled valleys and implications on design spectra E Faccioli and M Vanini Technical University of Milan, Italy Summary Evidence of complex site effects in two-dimensional geological configurations is illustrated first, and the related difficulties in interpreting ground motion observations are 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 one-dimensional propagation effects on the dominant frequencies of motion, and of two-dimensional effects on the amplification level and significant frequency band, are highlighted. Concerning design elastic spectra, estimates of response spectra on hard and soft soil are performed, using observed weak motion data and the empirical Green’s functions method. Comparison of results with Eurocode 8 spectra shows that the latter may not be conservative at periods larger than 1 s. The relatively large response at such is related to the geological configuration of the valleys, which appear to require a reasonably accurate definition of their two-dimensional geometry. Key words: earthquakes; site effects; alpine valleys; soil amplification; weak motion; two-dimensional numerical simulations; design spectra; empirical Green’s functions Prog. Struct. Engng Mater. 2003; 5:223–238 (DOI: 10.1002/pse.156) 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 by simple models based on a one- dimensional 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. One peculiarity of the European context is that it encompasses a broad range of exposure levels to earthquake hazard. While the protection of population centres from adverse site effects is a recognized concern in highly seismic regions such as Central/Southern Italy and Greece, this may not be the case for some zones of lesser seismicity where the near-surface geology can exert an unfavourable influence. For instance, in many alpine valleys in Italy and neighbouring countries the investment in buildings and infrastructure has been very large, but building design and land zoning regulations may not take adequately into account the nature and extent of adverse local effects. The lack of instrumental data from damaging earthquakes is in this case crucial because complex (two- or three-dimensional) site effects tend to occur on deeply incised, sediment- filled valleys and basins, where cities are located, or on steep topography, where some of the oldest settlements are found. From the land zoning point of view, earthquake-triggered slope failures constitute an induced hazard that may be typically overlooked in the alpine environment. It is instructive that the down-slope fall of some boulders was the apparent cause of the only two deaths occurred in the July 2001 Earthquake Engineering and Structural Dynamics Copyright & 2003 John Wiley & Sons, Ltd. Prog. Struct. Engng Mater. 2003; 5:223–238