Elastic moduli and seismogenic aspects of the Friuli upper crust.

We investigated the correlation between the spatial pattern of the elastic moduli and the seismicity in the central part of the Friuli region, the most active seismic zone in north-eastern Italy. The bulk modulus, the shear modulus and the Poisson ratio are calculated from Vp and Vp/Vs tomographic images. Seismicity correlates best with the shear modulus. Among the parameter derivatives, the shear modulus Laplacian is the best correlated parameter, negatively related to the released seismic energy. The zones with negative Laplacian are characterized by spatial shear modulus variation concavity, suggesting that the seismogenic volume could be formed by two zones. One zone is characterized by competent rocks with a great capability of storing strain energy and is a strong potential for severe earthquakes. The other zone, surrounding the former, is composed of less competent rocks, with low strain energy and low seismic energy release. These results suggest that the spatial pattern of a shear modulus could be useful to investigate the seismogenic behaviour, particularly in areas affected by complex tectonics, where it is difficult to recognize seismically active faults.