GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 2.1 267 The present study couples up-to-date procedures for PSHAwith the results of microzonation studies to achieve soil-specific seismic hazard maps for the westernmost sector of the Liguria region (northwestern Italy), an area where strong earthquakes (up to about magnitude 6.5) occurred in the past. In the present application, site amplification is quantified through pre- compiled, frequency-independent amplification factors defined as the ratio of the acceleration spectrum intensity (Von Thun et al. , 1988) for a 5% damping at the ground surface, SI S , to the acceleration spectrum intensity corresponding to the rock level, SI R :    SI S F S = –––    SI R Three different factors corresponding to different spectral ranges were defined. According to Priolo et al. (2019), the following period ranges are considered in the computation of SI S and SI R : 0.1 – 0.5 s, 0.4 – 0.8 s, and 0.7 – 1.1 s. The values of F S were determined via numerical ground response analyses based on appropriate soil models defined according to the geological features of the Liguria territory. The epistemic uncertainty in the soil parameterization was taken into account via Monte Carlo simulation (Barani et al. , 2013). All dynamic analyses were performed using the computer code Shake91 (Idriss and Sun, 1993) using a set of twelve real accelerometric recordings as input motion. They were selected according to the magnitude-distance scenarios controlling the hazard at short and long periods (Barani et al. , 2009). Following the Italian guidelines for seismic microzonation (SM Working Group, 2015), the values of F S were arranged into schedules (also termed as “abacuses”) as a function of soil fundamental frequency ( f 0 ) and time-averaged shear-wave velocity in the top 30 m ( V S,30 ). As an example, Figure 1 shows the geographical distribution of the F S values corresponding to the 0.1 – 0.5 s period range. Fig. 1 - Map of amplification factors corresponding to the 0.1 – 0.5 s spectral range.