GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 2.1 281 reaches depths greater than 30 meters the velocity structures are extrapolated, commonly with linear gradients. The choice of a linear gradient represents a simplified approach, and is the most common case treated in literature. A most realistic choice could be represented by velocity gradients which follow an exponential function. (Kaufman, 1953). One of the base equations used to obtain the distribution of shear-wave velocity (vs) in depth (z) is given by the following expression: vs(z)=v 0 (1+z) x (1) Where: - v 0 : surface shear-wave velocity - x: dependence of velocity in depth. x ∈ ]0,1[, which holds for each z≥0 This equation holds for granular media only, i.e., for media such as sands. In real cases, saturation level, fluid pressure and cementation (besides variations in form of sedimentation or exhumation) can significantly affect the exponent, which control the dependence of Vs on the depth. It is possible to find a consistent number of velocity gradients in literature, which follow the equation (1). In this study we collect the velocity gradients obtained for different alluvial basin such as the Lower Rhine Basin (Germany) (Budny, 1984; Hinzen et al. , 2002; Ibs-Von Seht, 1996; Parolai et al. , 2002) or other region characterized by widespread sedimentary cover (Boore et al. , 2003). For the upper 30 meters of the subsoil we consider a mean shear- wave profile composed by seven layers with typical properties of a sand deposit. Below the 30 meters we consider different type of exponential gradients found in literature and the linear gradient. The linear gradient is built assuming a seismic bedrock reaching a depth of 370 m (Fig.1). We perform simulations for each type of gradient using STRATA program, performing site response analysis in the frequency domain using time domain input option (the accelerogram of Friuli Earthquake, 6/5/1996, Mw: 6.5). In Fig.2 are shown the acceleration spectra obtained for each velocity gradient in Fig.1. We want to focus our attention on two points: first of all, there are differences for the maximum values for spectral acceleration. Moreover, the different bedrock depths (given by the threshold of 800 m/s) influence the spectral response. In general, the type of velocity gradient is of paramount importance for ground response analysis and, consequently, for seismic hazard assessment. Fig.1 - Velocity Gradients which follow the linear and exponential functions.