Characterization of crustal physical properties in the Montello-Collalto area (eastern Southern Alps, Italy)

The Montello–Collalto area is located along the outer front of the eastern Southern Alps (Italy), within a fold-and-thrust belt that has been active since the Middle Miocene (Picotti et al., 2022). The region is characterized by a medium-to-high seismic hazard, as demonstrated by historically significant earthquakes such as the 1695 Asolo event (Mw 6.5). Despite this, the causative fault system remains poorly constrained, mainly because most tectonic structures, including the Montello thrust system, are buried beneath recent sediments, and the overall seismicity rate is generally low

At a depth of approximately 1.5 km within the Montello anticline, an Underground Gas Storage (UGS) facility is in operation. The site is continuously monitored by the National Institute of Oceanography and Applied Geophysics (OGS) through both regional and dedicated local seismic networks (Priolo et al., 2015). By collecting the seismological data acquired over the years from these two networks and other passive seismic experiments, Cipressi et al. (2025) recently compiled a new uniform seismic catalog for the area. It includes 4802 earthquakes (-0.9 ≤ ML ≤ 3.9) that occurred between 1977 and 2023, all relocated using the same code (NonLinLoc, by Lomax et al., 2001) and velocity model (Romano et al., 2019).

To better characterize the 3D seismic velocity structure of the area, a new velocity model was developed, based directly on laboratory measurements performed on rock samples representative of the local stratigraphic sequence. Through a fieldwork conducted in the study area, a total of 22 samples were collected and subjected to VP and VS measurements at ETH Zürich using the pulse-transmission method (Birch, 1960). Overall, the measurements were performed under varying confining pressures, during both loading and unloading phases, ranging from 5 MPa to 200 MPa to simulate different depth conditions.

The laboratory-derived values were scaled for better corresponding to the lithological volumes and implemented within a dedicated 3D geological model of the study area, based on the structural interpretation by Picotti et al. (2022) and constructed using Midland Valley’s 3D Move software. This approach allows for a detailed and physically constrained characterization of seismic velocities in the upper ~10 km of the crust, which represents the depth range most relevant for the UGS monitoring.

The newly developed 3D velocity model will be tested and validated by relocating the seismic events included in the updated seismic catalog (Cipressi et al., 2025). Through the analysis of the travel time residuals we will assess whether velocity models derived from geological and laboratory data can effectively constrain seismic velocities and improve earthquake locations. Ultimately, this approach may also help refine the current geological interpretation of the area and improve understanding of the seismic behaviour of the main seismogenic structures.