Short Note 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 func- tions. Results obtained from earthquake data are then compared with ambient noise analysis.Azimuthaleffectsintheseismicresponseareobservedlocallyforwavescom- ingfromtheeast-southeast,alignedwiththelongitudinalaxisofthevalley.Theseismic responsesevaluatedatthemid-valleyandatthevalleyedgearethenconvolvedwiththe 1976Mw 6:4Friuli 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 responsibleforanenhancementoftheinputmotionofanaveragefactorofabout4inthe frequencyrangefrom2to8Hz.Theelasticdemandfromthecurrentnationalregulation 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 VS30 site classification. Online Material: Figuresshowinglocationsofhistoricalearthquakes,distributionof fundamental frequency of vibration, waveforms of the MD 5.1 Kobarid earthquake, S-wavevelocityversusdepth,andGITandRFamplificationmedianvaluesandspectra; tables of seismic station locations and soil type, and of local and regional earthquakes.