Explosion, earthquake, and ambient noise recordings were collected at the eastern edge of a Pliocene sediment-filled valley, nearby the town of San Casciano dei Bagni (Tuscany, Central Italy). The anomalous amplification of amplitudes (as high as a factor of 20 between sediment and hard-rock sites) shown by classical spectral ratios were compared with results obtained by using non-reference-site techniques, like that of Nakamura (1989) and its receiver-function-like extension.
The explosion data were collected in June 1990, as part of a crustal refraction experiment carried out in the Monti Vulsini volcanic area. A linear array of 13 digital stations was deployed orthogonally to the eastern border of the Radicofani graben: 2 stations were set up on a bedrock outcrop, whereas the remaining 11 were deployed on soft sediments, 100 m apart, along a straight line toward the center of the valley.
During a second part of the study, two stations were left in the field to record local and regional seismicity, acquiring data from more than 150 events from a seismic swarm that occurred in the Monti Vulsini volcanic area in February 1992 with a maximum magnitude MD = 3.8. Spectral ratios were computed on the 41 strongest events, showing a seismic response consistent with that computed using explosion data, at least in the 0.5- to 4-Hz frequency band. Non-reference-site techniques were employed to analyze earthquake and noise data: although it is clear that the horizontal/vertical (H/V) spectral ratios describe some intrinsic properties of sites (noise and earthquake data give equivalent results), in our case, they failed in recognizing both resonances and amplification levels.
Large amplifications shown by classical spectral ratios were probably due to basin-induced surface waves and 3D resonances. Seismic waves recorded by a small (50m radius) 2D array deployed within the valley provided instrumental evidence for the existence of waves diffracted locally on lateral heterogeneities.