EXPLOSION, EARTHQUAKE, AND AMBIENT NOISE RECORDINGS IN A PLIOCENE SEDIMENT-FILLED VALLEY - INFERENCES ON SEISMIC RESPONSE PROPERTIES BY REFERENCE-AND NON-REFERENCE-SITE TECHNIQUES

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 amp lification of amplitudes (as high as a factor of 20 between sediment a nd hard-rock sites) shown by classical spectral ratios were compared w ith results obtained by using non-reference-site techniques, like that of Nakamura (1989) and its receiver-function-like extension. The expl osion data were collected in June 1990, as part of a crustal refractio n 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 o utcrop, 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 re cord local and regional seismicity, acquiring data from more than 150 events from a seismic swarm that occurred in the Monti Vulsini volcani c area in February 1992 with a maximum magnitude M(D) = 3.8. Spectral ratios-were computed on the 41 strongest events, showing a seismic res ponse consistent with that computed using explosion data, at least in the 0.5- to 4-Hz frequency band. Non-reference-site techniques were em ployed 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 result s), in our case, they failed in recognizing both resonances and amplif ication levels. Large amplifications shown by classical spectral ratio s were probably due to basin-induced surface waves and 3D resonances. Seismic waves recorded by a small (50-m radius) 2D array deployed with in the valley provided instrumental evidence for the existence of wave s diffracted locally on lateral heterogeneities.