Complex seismic site effects in sediment-filled valleys and implications on design spectra

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.