The alluvial plain on the northern shore of Lake Garda (Italy) as a case study for physics-based numerical simulations of site effects.

The role of 2-D and 3-D geometry in the seismic response of alluvial valleys and sedimentary basins can be evidenced by physics-based numerical simulations of seismic wave propagation in heterogeneous media. The present work focuses on the 5 km wide valley on the northern shore of Lake Garda in the Italian Alps. A recent study carried out in this area has shown that amplifications of earthquake ground motion up to 10 in the frequency range of engineering interest (0.5-10 Hz) are possible at sites inside the valley in respect to a rock site. To understand the origin of the observed site response, which 1D stratigraphic effects alone cannot explain, we used the available geological and geophysical data and built a 3D digital structural-geophysical model. The used data consist of seismic reflection profiles, interpreted geological sections and borehole measurements from existing literature, as well as data from newly conducted measurement campaigns of microtremors, shear wave velocity profiles and gravity. In the present work, we demonstrate the efficiency of the resulting 3D model in simulating the ground motion variability by a quantitative comparison between the empirical and the numerically evaluated amplification functions at a number of sites. In particular, we consider the amplification functions evaluated from earthquake ground motion recordings at 19 sites, where a temporary seismological network operated between 2019 and 2021. We evaluate the numerical amplification functions from physics-based numerical simulations of vertically emerging plane waves in the digital 3-D model. In order to perform the numerical simulations we used the 3-D spectral-element and frequency-wave number hybrid method, that is implemented in the latest versions of the open-source software SPECFEM3D Cartesian. The study confirms that the area is susceptible to combined 1D to 3D site effects generated by the peculiar geometry of the deposits composing the basin. The validated 3D model could provide a basis for the calculation of earthquake scenarios in the area.