Local site response in an Alpine Valley from recorded earthquakes and ambient noise analysis

Local response is estimated within the sediment fill of the Tagliamento River Valley (THV), northeastern Italy, using about 90 weak-to-medium local and regional earthquakes. Ground motion is recorded at six stations deployed in a linear array across the valley. Site amplification factors are determined using the standard spectral ratio, the generalized inversion technique, and evaluations of receiver functions. Results obtained from earthquake data are then compared with ambient noise analysis. Azimuthal effects in the seismic response are observed locally for waves coming from the east-southeast, aligned with the longitudinal axis of the valley. The seismic responses evaluated at the mid-valley and at the valley edge are then convolved with the 1976M w 6.4 Friuli mainshock recorded at accelerometric station TLM1, located close to the array. The local response at the valley edge, in the town of Cavazzo Carnico, is responsible for an enhancement of the input motion of an average factor of about 4 in the frequency range from 2 to 8 Hz. The elastic demand from the current national regulation for a 475-year return period and specific for Cavazzo Carnico is overcome by the 5%-damped pseudoacceleration response spectrum estimated at the valley edge. The underestimation of the Italian code is mostly due to a 2D amplification effect at the valley edge rather than a simple inadequacy of the V S30 site classification.