Three-dimensional V-P and V-P/V-S models of the upper crust in the Friuli area (northeastern Italy)

3-D images of P velocity and P- to S-velocity ratio have been produced for the upper crust of the Friuli area (northeastern Italy) using local earthqu ake tomography. The data consist of 2565 P and 930 S arrival times of high quality. The best-fitting V-P and V-P /V-S 1-D models were computed before the 3-D inversion. V-P was measured on two rock samples representative of t he investigated upper layers of the Friuli crust. The tomographic V-P model was used for modelling the gravity anomalies, by converting the velocity v alues into densities along three vertical cross-sections. The computed grav ity anomalies were optimized with respect to the observed gravity anomalies . The crust investigated is characterized by sharp lateral and deep V-P and V-P /V-S anomalies that are associated with the complex geological structu re. High V-P /V-S values are associated with highly fractured zones related to the main faulting pattern. The relocated seismicity is generally associ ated with sharp variations in the V-P /V-S anomalies. The V-P images show a high-velocity body below 6 km depth in the central part of the Friuli area , marked also by strong V-P /V-S heterogeneities, and this is interpreted a s a tectonic wedge. Comparison with the distribution of earthquakes support s the hypothesis that the tectonic wedge controls most of the seismicity an d can be considered to be the main seismogenic zone in the Friuli area.