Effects on local seismic response due to underground caves though a geophysical-validated numerical modelling

In Mediterranean areas, many countries have several natural and anthropic underground caves, which have been used for different purposes over time and they can represent a potential geo-hazard for both buildings and infrastructures, especially in the historical centers hosted in seismically active areas (Fabozzi et al., 2024; Faraone et al., 2023). The main source of cavity-related risk is due to two different phenomena: i) stress induced by seismic shaking can exceed the rock strength of around the caves, causing a vault collapse; ii) seismic waves propagating inside the caves can be modified in terms of frequency, amplitude and duration, influencing the local seismic response with respect to a free field bedrock (Fabozzi et al., 2024). Nowadays, the impact of underground caves and the prediction of their related effect on seismic motion are generally not taken into account for Seismic Microzonazion (SM) studies. In the literature, a 2D model is proposed to study the propagation of shear seismic waves caused by a single arch-shaped cave in homogeneous rock, considering their size and depth (Lancioni et al., 2016). Furthermore, Faraone et al. (2023) performed a local seismic response study using 2D linear numerical analysis on urban cavities, considering different soil shear wave velocities (Vs), sizes, depths, and lining conditions. This study proposes an improvement in understanding the mechanisms of interaction between surface seismic waves and natural caves under free-field conditions. It also seeks to categorize local seismic response based on their physical and mechanical characteristics. A statistical study performed on a dataset of sinkholes inventoried in the Maltese Archipelago allow to identify the relevant morphometric parameters: predominant geometries, size classes, depths and the lithologies. Specifically obtained geological sections reinstituted the average thicknesses of the geological units have been obtained. The combination of all these variables allowed to setup a parametric analysis via numerical modelling, based on the most representative combinations. The parametric analysis includes a two-dimensional numerical modeling (2D) under Linear Elastic conditions through the finite difference code FLAC 2D (Itasca Consulting Group). The same methodology is applied to “validating cases (VC)”: sites where geophysics data are available. These sites are Ghar il-Latnjia and Ghar il-Kbir Caves in Malta and Battifratta and Grotta Scura Caves in Italy. Finally, once the simulations have been validated, they will be applied to a “demonstrating case (DC)”, where no geophysical data are available. In this case, the DC selected is the Tal-Mixta Cave in Gozo. The challenge of this study is to develop seismic intensification charts in order to standardize the seismic response of natural cavities under free-field conditions, considering different geometries, depths, and size classes.