Integrated crustal model beneath the Po Plain (Northern Italy) from surface wave tomography and Bouguer gravity data

To obtain a 3-D crustal density and shear-wave velocity structure beneath the Po plain, we exploit seismic records gathered from 2006 to 2014 and Bouguer gravity data assembled for the last estimation of the Italian Geoid. 2-D maps for both Love and Rayleigh fundamental mode at periods between 4 and 20 s are obtained applying a tomographic inversion. The defined local dispersion curves are then jointly inverted using a linearized scheme to obtain a 3-D isotropic shear-wave velocity model across the Po plain region. The model, transformed into density through a priori velocity-density relationships, is then the input of the Sequential Integrated Inversion algorithm, which enables us to recover a new 3-D density-shear wave velocity coupling and inferences on the lithology and tectonics. Low and fast S-wave velocities are highlighted for the shallow Pliocene–Quaternary sediments along the foredeep, in front of the Northern Apennines, and for the presence of limestone units in the upper crust, respectively. Whereas sediment trends seem to be consistent with the results obtained, the Mesozoic carbonates, which are inherently characterized by high variability, are less resolved. A major result is the recovery of a high speed (3.3 km/s) - density (2.2 kg/m3) structure in the upper crust (6–10 km) localized beneath the arcuate Po plain thrust front expanding from the external margin of the Ferrara arc toward the Alps and the Adriatic Sea. At the boundaries of this brittle body, we locate earthquakes of the Emilia 2012 seismic sequence and the historical seismicity. Mapping lateral discontinuities in density and shear wave velocity could provide insights in defining strengthening and weakening zones, and in focusing on transition zones often prone to earthquakes.