Combined Geophysical Approaches for Urban Subsurface Exploration: A Case Study from Messina, Italy

In this study, we performed integrated geophysical surveys in the historical center of Messina, focusing on the area surrounding the Cathedral, where urban stratigraphy is strongly influenced by both natural and anthropogenic processes. The city of Messina, southern Italy, is characterized by high seismic hazard and complex near-surface conditions. Messina has been repeatedly struck by destructive earthquakes over the last centuries, most notably the M7.1 event of 1908, which caused near-total destruction. Reconstructions following these earthquakes generated thick and laterally non-uniform anthropogenic deposits (rubble and debris) that, combined with vertically heterogeneous stratigraphy, might pose significant challenges for accurate subsurface characterization and site response analysis. Ambient noise data were analyzed using the Horizontal-to-Vertical Spectral Ratio technique to estimate fundamental resonance frequencies and delineate major impedance contrasts. Results from HVSR measurements revealed a predominant fundamental resonance peak around 1.0 Hz, with amplitude factors between 4–6, consistent with the impedance contrast between alluvial sediments and the underlying metamorphic basement.

To examine the spatial distribution, the HVSR data were organized along five survey profiles. The corresponding two-dimensional cross-sections were generated by interpolating more than five HVSR measurements for each profile. In addition, high-frequency peaks (>30 Hz) were detected and mapped laterally for more than 40 m, strongly suggesting the presence of heterogeneous anthropogenic layers formed after the 1908 earthquake, highlighting the significant role of shallow debris deposits in conditioning site response.

Active and passive surface-wave methods, including Multichannel Analysis of Surface Waves (MASW), Extended Spatial Autocorrelation (ESAC) and (f-k) approaches, were employed to retrieve shear-wave velocity profiles at different depths. MASW results identify a 2–3 m low-Vs layer (150–200 m/s) overlying 350–450 m/s alluvial deposits, while array analyses (ESAC, f-k) extend the investigation depth to ~60 m. Joint HVSR–dispersion inversion constrains the main impedance contrast at 90–100 m, marking the transition to the metamorphic basement. Differences between MASW and joint inversion models highlight the importance of multi-method approaches when anthropogenic stratigraphy is present.

The joint analysis allowed the identification of key stratigraphic interfaces, the recognition of laterally variable anthropogenic fills and deposits, and the estimation of the main discontinuities within the uppermost layers. Importantly, the 2D HVSR cross-sections enabled mapping lateral variations in resonance frequencies, highlighting the spatial extent of post-1908 anthropogenic deposits. These findings demonstrate the effectiveness of a multi-method approach in resolving shallow subsurface complexity in highly urbanized areas.