GNGTS 2016 - Atti del 35° Convegno Nazionale

216 GNGTS 2016 S essione 1.2 THE GEOPHYSICAL STRESS MAP vs A GPS INFERRED STRAIN-RATE MAP: A COMPARATIVE ANALYSIS B. Mastrolembo, A. Caporali Dipartimento di Geoscienze, Università di Padova, Italy Introduction. The traditional approach in the measurement of stress orientation consists mainly of borehole breakouts inwells and of fault plane solutions. The instantaneous deformation in the neighborhood of the well or the fault zone is measured. In Italy the presence of different tectonic regimes that change within short distances is such that the predominant wave-length of the deformation pattern is ~150 km (Pierdominici and Heidbach, 2012). On the other hand, the measurement of the horizontal gradient of the velocity of GNSS sites is also related to the present day strain rate. The geophysical data provide the direction of stress of a very localized area, whereas the geodetic data describe the strain rate in an area covered by a sufficiently large number of GNSS sites. For sufficiently dense GNSS networks in areas subject to active deformation of a relatively large wavelength, the average strain rate from GNSS geodesy and the stress directions from geophysical data should be comparable, provided that the deformation which is inferred on the surface by geodetic methods remains constant at the average depth of the wells or of the hypocenter. The principal directions of the strain rate tensor coincide with those of the stress rate, in a plane stress approximation, assuming that stress and strain are linearly dependent, i.e. elastic rheology. This assumption is acceptable if we confine to the upper part of the crust. In this work we present a direct comparison of principal direction of geophysical stress and geodetic stress rate. We use the stress data of Montone and Mariucci (2016) and the strain rate field based on the GNSS velocities computed at weekly updated at the University of Padova (Caporali et al. , 2016), using EUREF processing standards (Bruyninx et al. , 2013). After checking that the distribution of the GNSS velocities provide a redundant coverage of the area of interest, the geodetic strain rate tensor is interpolated at the location of borehole breakouts and fault plane solutions in the Italian stress map. We focus on sites of class D or better. Following the ZS9 zonation of Meletti et al. (2008) we compare the principal direction of geodetic and geophysical stress in homogeneous zones: three are mostly compressive (Friuli, Emilia and northern Sicily), two are extensive (Central and Southern Apennines) and one is prominently strike slip (Gargano). All have a dimension smaller than the typical wavelength of the dominant deformation pattern. We show that the principal directions agree within the given tolerance for all the six areas. The residuals have a nearly random distribution about zero. In some cases of small number of data, we find asymmetries. However attempting an interpretation is unjustified by the quantity of supporting data. We conclude that there is, in all the analyzed areas, an excellent agreement between the two totally independent data on stress orientation. More work is necessary to extend the analysis to other areas, such as the outer Apenninic front in the Ancona area, for which we anticipate a mixture of compressional and transcurrent stress regime. Input data. In 2016, Montone and Mariucci released an updated stress map that includes seismicity, breakout of deep wells, fault and seismic sequence data for the Italian territory (Fig. 1). Present-day stress indicators are generally grouped into earthquake focal mechanisms, borehole breakouts, drilling-induced fractures, hydraulic fractures, overcorings and borehole slotters, petal-centreline fractures and geological indicators (Montone and Mariucci, 2016). The new stress map, released in 2016, shows minimum horizontal stress orientation (Sh min ) coming from different stress indicators such as borehole breakouts, earthquake focal mechanisms, active faults and formal inversion of seismic sequences. A total number of 630 data were included into their present-day stress map, with a good coverage of the most seismically active areas on the Italian territory. According to the WSM Project rules (http://www.world-stress-map.org; Heidbach et al., 2010) data from 855 entries were quality-ranked as five categories identified