Crustal imaging of the seismic velocity structure in the Emilia region

The 2012 Emilia seismic sequence in the central sector of the Ferrara arc included the 20 May (Mw 6.1) and 29 May 2012 (Mw 6.0) mainshocks, followed by thousands of aftershocks, and ruptured thrust faults belonging to the Ferrara and Mirandola systems buried beneath the Po Plain (Carannante et al., 2015). Several studies have demonstrated the exceptional value of this sequence for crustal imaging: refined aftershock relocations highlighted the activation of adjacent blind thrusts and the structural complexity of the Apennines frontal belt (Govoni et al., 2014), while additional analyses revealed significant lateral heterogeneity along the Ferrara arc (Chiarabba et al., 2014). The dense permanent/temporary network deployed during the crisis produced one of the most complete seismic datasets for northern Italy.

Within this framework, we construct a new three-dimensional a-priori P-wave velocity model for the Emilia–Romagna region, spanning 10–13°E and 44–46°N and parametrised on a 15-km horizontal grid with 3-km vertical spacing. The workflow follows a multi-dataset integration strategy, in which velocity–depth functions are extracted at each node of a horizontal grid and lateral continuity is ensured through spatial smoothing. The model assimilates multiple complementary datasets: Vp control points from the 3-D Po Basin model of Molinari et al. (2015), regional geological cross-sections from the ER3D model (Klin et al., 2019) used as qualitative constraints on basin and thrust geometry, crustal and lithospheric information from published tomographic models (Di Stefano et al., 2011; De Gori et al., 2014) together with the recent adjoint tomography model of the Italian lithosphere (Im25; Magnoni et al., 2022), and structural and seismogenic constraints derived from analyses of the 2012 sequence (Govoni et al., 2014). This multi-source integration produces a geologically coherent three-dimensional starting model that better represents the strong lateral variations of the Po Plain than conventional one-dimensional or poorly constrained three-dimensional initial models.

The resulting model (the a-priori model) is employed as the initial structure for 3-D travel-time tomography, implemented through an iterative inversion approach adapted to the characteristics of the Emilia region. High-quality P- and S-wave arrival times recorded by the seismic network operating during the 2012 sequence offer favourable ray coverage especially in the upper and middle crust. This helps mitigate typical limitations introduced by sharp lateral velocity contrasts and irregular station spacing, improving the reliability and resolution of the final tomographic images.

This work contributes to refine seismic imaging and hazard assessment in the Po Plain. By demonstrating the advantages of constructing a detailed a-priori velocity model in a structurally complex region, it highlights the importance of integrating multiple geophysical datasets to obtain a stable foundation for tomographic inversion. A refined starting model enhances the ability to resolve lateral heterogeneities within the sedimentary basin and better define the geometry of deep thrust systems. The resulting framework supports future investigations of ground-motion amplification, fault interaction and crustal structure along the Apennines front in one of the most industrialised and densely populated regions of northern Italy.