Stress and strain-rate fields: a comparative analysis for the Italian territory

We present a comparative analysis between the contemporary Italian crustal stress map and the strain-rate field inferred from GPS velocities. We use the most complete and updated stress map of Italy which consists of 630 data mainly from borehole breakouts, earthquake focal mechanisms, and active faults with an unprecedented spatial coverage of several tectonic areas within the Italian territory. Stress and strain rate are physical quantities, linked by a causal relationship, with parallel eigenvectors in case of a perfectly elastic mean. Comparing surface geodetically inferred strainrate orientations with the stress measured in the crust allows a quantitative analysis of the relation between surface deformation and deep tectonic processes acting at the different depths in the brittle crust. Data from over 500 GPS stations with a minimum time span of 2 years are weekly analyzed, according to the international standards, to estimate a velocity field aligned with the ITRF frame (IGb08). We use a least-squares collocation algorithm to interpolate the horizontal GPS velocities and estimate the strain-rate eigenvectors at the position of those stress data surrounded by a sufficient number of GPS stations. The stress data set consists of borehole breakout (305, A-D quality) and earthquake focal solution (392, C quality) measurements. We identify five tectonic provinces covered by a large population of GPS sites and rather homogeneous stress orientations: three of them, Friuli, Emilia, and outer Apennines area undergoing compression, while central and southern Apennines are subject to extension perpendicular to the belt axis. We compute the difference in azimuth of minimum horizontal stresses between each breakout or focal mechanism measurement, and the geodetically inferred direction. Our analysis shows that the geophysical and geodetic measurements predict in most cases (~60%) the same orientation for the crustal stress and surface strain-rate tensors, within one standard deviation. The largest azimuthal differences (>80°) are observed in the outer Apennines area for a few stress data from focal solutions of relatively deep events. This might suggest that the observed strainrate at the Earth surface is decoupled from deformation processes at crustal depth.