Physics-based simulation of 3D seismic site effects: Case study of the lower Sarca Valley (Trentino, Italy)

We can investigate the role of 2D and 3D geometry of sedimentary cover and bedrock topography in the seismic response of alluvial valleys and sedimentary basins by physics-based numerical simulations of seismic wave propagation in heterogeneous viscoelastic media. In this context, the present work focuses on the simulation of earthquake ground motion in the lower part of the Sarca Valley, a 5 km wide valley on the northern shore of Lake Garda in the Italian Alps. A recent study has shown that in this valley the amplitude of earthquake ground motion in the frequency range of engineering interest (0.5–10 Hz), can be amplified by a factor of up to 10 compared to rock sites. To investigate the origin of the observed amplification values, we numerically simulate the ground motion in the valley using a 3D digital structural-geophysical model created from the available geological and geophysical data. These 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. The level of detail that results from the constructed 3D digital model is sufficient to simulate the valley's response within the frequency range up to 5 Hz. Although the simulations performed in this work do not cover the entire frequency range of interest—extending up to 10 Hz—they nevertheless encompass the peak frequencies observed in the amplification functions at internal points within the valley. To perform the numerical simulations, we used the 3D spectral-element and frequency-wave number hybrid method, which is implemented in the latest versions of the open-source software SPECFEM3D Cartesian. We assess the capabilities of both the 3D and 1D modeling approaches to reproduce the observed seismic response within the frequency range of 0.5 Hz–5 Hz, by performing a quantitative comparison between empirical amplification functions and those obtained numerically at a selected set of locations. In particular, we considered the amplification functions evaluated from earthquake ground motion recordings at 19 locations, where a temporary seismological network operated between 2019 and 2021. The results confirm 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 provides a basis for the calculation of earthquake scenarios in the area with accurate consideration of site effects from 0.5 to 5 Hz. An original aspect of this work consists in the introduction of a nine-component descriptor for site response, referred to as the site-to-site transfer function for vertically incident plane waves.