Site effect of the strong-motion site at Tolmezzo-Ambiesta Dam in Northeastern Italy

A spectral analysis of strong-motion data and ambient noise at the Tolmezzo-Ambiesta dam accelerograph (TLM1) site is performed to explain the high horizontal peak acceleration of 0.36g recorded during the Mw 6.4, 6 May 1976 Friuli (northeastern Italy) earthquake. The spectral response of the accelerographic site is estimated from the mainshocks and aftershocks of the 1976 seismic sequence by different techniques. In addition, new seismic data are used to characterize the nearsurface S-wave velocity distribution versus depth at TLM1 by means of an analysis of the phase-velocity dispersion of Rayleigh waves. An average, spectral amplification curve is computed for TLM1, whose mean value is at about 1.5. It features two main lobes of amplification with peaks at 2–3.5 Hz and 6–7 Hz that are explained, by the horizontal-to-vertical spectral ratio (HVSR) of noise measurements, as the effects of the vibration of a nearby relief and the dam-reservoir system, respectively. When the site response is evaluated from earthquake recordings, the frequency of resonance due to near-surface S-wave velocity profile at about 5 Hz is strongly attenuated in comparison with the 6–7 Hz peak due to the dam-reservoir system. Finally, by filtering the site effects of the 1976 record, the acceleration peak is reduced from the original value of 349.99 cm/sec2 to 188.76 cm/sec2.