Sequential Integrated Inversion of tomographic images and gravity data: an application to the Friuli area (north-eastern Italy)

The three-dimensional pattern of elastic moduli (bulk modulus, Young modulus, shear modulus) of the upper crust (0-10 km depth) has been determined in the Friuli area (north-eastern Italy) from the 3D Vp, Vp/Vs and density structures. Firstly, 3D Pwave velocity and P to S velocity ratio were modeled by joint inversion for hypocentres and velocity structure. Then, we apply the tomographic inversion method of Sequential Integrated Inversion (SII) to recover the three dimensional density structure. The pattern of the elastic moduli is characterized by marked lateral and depth variations that reflect the geologic-structural heterogeneity of the area, produced by the superposition of several tectonic phases with different orientations of the principal axes of stress. The bulk (K), Young (E) and shear (G) moduli image a high rigidity body with an irregular shape, at 4-8 km depth. The body is characterized by G >= 3.2·1010 N·m-2, K >= 6.8·1010 N·m-2 and E >= 8.4·1010 N·m-2 and is associated to platform limestones and dolomitic rocks. The seismicity is mainly located along the sharp variations of the moduli pattern, in or adjacent to high rigidity zones. The most severe earthquakes (ML between 4.5 and 6.4), occurred in the study area from 1976 to the present day, are located in a transition zone from high to low rigidity patterns. Our interpretation is that the elastic moduli variations, closely related to variability in rock mechanical properties, influence the occurrence of earthquakes by processes of stress concentrations. The values of the elastic moduli recently obtained from laboratory measurements on the main lithologic units fall in the middlehigh range of the values obtained with the present investigation.