Spatial variation in stresses in peninsular Italy and Sicily from background seismicity

We have retrieved the stress tensor orientation in the Italian region, applying the FMSI technique (Gephart, J., 1990b. FMSI: a FORTRAN program for inverting fault/slickenside and earthquake focal mechanism data to obtain the regional stress tensor. Comput. Geosci. 16, 953-989) to fault plane solutions of 298 small earthquakes (2.5 < M(d) < 4.8) recorded by the National seismic network of the Istituto Nazionale di Geofisica in the period 1988-1995. First, we verified that the stress inversion procedure, generally applied to a large number of focal mechanisms of earthquakes recorded in a local seismic network, could be successfully used in Italy, using background seismicity recorded over large (similar to 100-200 km) regions. Then, comparing the stress inversion results with other stress indicators, we iind that they are geologically sound for many regions in Italy, even if the misfit values are relatively large. We also find that the largest misfit values are determined by a few earthquakes (only 10-20% of the total number). Excluding these earthquakes from inversion, the average misfit becomes much lower (by similar to 30-40%). The results obtained in this study confirm the large variations of the stress field over a relatively short distance in Italy, but also confirm the existence of regions extending for a few hundred kilometres within which the stress is homogeneous. Among these, most of the Apennines are dominated by extension perpendicular to the trend of the mountain belt. Only in the northern Apennines is there any clear distinction between an area of extension in the inner portion of the belt and an area under horizontal compression along the Adriatic margin of the belt. More rapid variations occur around Calabria and Sicily, as expected because of the complexities linked with the Tyrrhenian subduction and with active volcanism. In the Ionian Calabrian coastal region, there are hints of active NW-SE compression, although constrained by a few data, whereas in the Messina Strait, we observe similar to E-W extension and a vertical sigma(1). In northern and western Sicily, our results indicate N-S to NW-SE compression, roughly consistent with the general convergence between the European and the African plates. (C) 2000 Elsevier Science B.V. All rights reserved.