Near-Field Response of a 1D-Structure Alluvial Site

We present a case history of the complicated strong-motion responses ata site with an almost 1D structure (Buia) in the near field of four Mw 5.2–6.0 earthquakesin the Friuli Plain of Italy. In one case (15 September 1976, 09:21 UTC,Mw 6.0), the 47-m alluvial cover of Buia experienced spectral amplitudes lower thanits reference site on rock due to the different back azimuths from the source. Thelargest peaks at Buia occurred during an Mw 5.6 event. For practical purposes, wetested (1) subjectivity when preparing the 1D models (using a blind test with threeexperts); (2) 1D linear, linear-equivalent, or nonlinear modeling in common practice;and (3) the current seismic regulations. We also used noise recordings and the weakmotionvelocities recorded for local events (with digital triplets on the top and thebottom of the alluvial cover). Buia’s response to weak events was more stable thanits response to strong ones. A simple 1D model (five-layer) under linear conditionswith good knowledge of the propagation velocities was able to reproduce the frequenciesof the two relative strong-motion maxima with an average amplitude underestimationof 1.5. The nonlinear approach simulated the two recorded maxima with slightshifts in frequency. Because the disaggregation of the seismic hazard showed thatthe four destructive shocks studied represent the magnitude and distance ranges thatdominate the hazard of Buia, we can conclude that the response spectra of thenew Italian (and European) rules were sufficiently preventive, notwithstanding theaforementioned complications in the near field.