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

We present a case history of the complicated strong-motion responses at a site with an almost 1D structure (Buia) in the near field of four M w 5.2-6.0 earthquakes in the Friuli Plain of Italy. In one case (15 September 1976, 09:21 UTC, M w 6.0), the 47-m alluvial cover of Buia experienced spectral amplitudes lower than its reference site on rock due to the different back azimuths from the source. The largest peaks at Buia occurred during an M w 5.6 event. For practical purposes, we tested (1) subjectivity when preparing the 1D models (using a blind test with three experts); (2) 1D linear, linear-equivalent, or nonlinear modeling in common practice; and (3) the current seismic regulations. We also used noise recordings and the weakmotion velocities recorded for local events (with digital triplets on the top and the bottom of the alluvial cover). Buia's response to weak events was more stable than its response to strong ones. A simple 1D model (five-layer) under linear conditions with g...