Stress field of Italy — Mean stress orientation at different depths and wave-length of the stress pattern

Abstract

We have studied the stress pattern of Italy using a dataset with 590 data records from the World Stress Map database release 2008 and 106 new data records to test the hypothesis the mean orientation of maximum horizontal stress SH is different at different depth sections. For this, we split the dataset into a shallow (0–6km) and a deep (6–40km) depth section. For the data analysis we used a new statistical tool that calculates the mean SH orientation on a 0.2° grid. The tool takes into account the distance to each grid point, number and quality of the data records within the search radius, and the radial distribution. The result is a smoothed Italian stress map that displays both; the mean SH orientation and the wave-length of the stress pattern. The stress pattern does not vary in depth except for two areas (Sardinia and southern Apulia). Therefore stress data from different depths can be used to estimate the mean SH orientation and the wave-length of the stress pattern in Italy. Furthermore, the smoothed Italian stress map reveals that most of Italy has short wave-length stress patterns (<150km). This indicates that the stress field is not controlled by first-order stress sources of plate tectonics, i.e. the convergence of Africa with respect to Eurasia, but that second-order stress sources such as topography, density, strength contrasts, and major faults systems are of great importance. In four regions (western part of the Alps, northern Tuscany, northern Adriatic Sea, Calabria and eastern part of Sicily) the wave-length is <100km. High values of the circular variance of the mean SH orientation observed here are driven by third-order local stress sources, such as basins or local neotectonic structures.