A GIS‑based approach for planning investigations in Seismic Microzonation studies of Level 3: methodology and application in the Etnean area, Italy

Seismic Microzonation studies of Level 3 (SM3) require a detailed and spatially homogeneous characterization of subsurface conditions, typically obtained by integrating geognostic and geophysical datasets. However, the distribution of existing investigations (boreholes (S), Multichannel Analysis of Surface Waves (MASW), Down‑Hole test (DH), Horizontal to Vertical Spectral Ratio (HVSR)) is often irregular and clustered and, consequently, insufficient to represent the geological and geotechnical variability of the entire study area. This is particularly true in those geological contexts, such as volcanic settings, characterized by strong lateral and vertical heterogeneities. This paper presents a reproducible GIS‑based approach for designing an optimal investigation plan for SM3 through the construction of a regular network of Control Points (CPs). The methodology is entirely implemented using open‑source GIS tools and consists of: (i) generating two regular grids (size: 500 m and 1000 m), (ii) extracting and cleaning grid centroids by removing those located outside SM3 areas, (iii) integrating additional CPs where necessary, and (iv) associating each CP with pre‑existing investigations within a significance‑based distance threshold and within the same SM1 stable or instable zone (stab/instab). The method is tested in the Etnean area (Sicily, Southern Italy), where complex volcanic architectures strongly influence the local seismic response. Results indicate that the CP network allows for a rapid identification of data gaps and supports a rational design of new investigations, ensuring homogeneous spatial coverage and improving the reliability of the SM3 subsurface model. This GIS‑based framework provides a transparent and fully reproducible workflow that can be applied to any SM3 municipality studies at national scale.