GNGTS 2015 - Atti del 34° Convegno Nazionale

92 GNGTS 2015 S essione 1.2 (4) This is accomplished by repeating, at each point, the calculation of the shear strain rate (Eq. 4) for values of d 0 spanning a range, e.g. 10-100 km, and choosing the value that maximizes the shear strain rate. A check is made that at least three GNSS sites exist within the selected scale distance from the computation point. Combining geodetic and seismologic data. Once a reasonably detailed map of the deformation rates has been obtained on a regional scale by geodetic method, it is natural to ask how and towhich extend this deformation does correlate with the statistical seismicity of the area. This theme is of particular relevance at this time, when finite element models of deformation at depth and at the surface are becoming available for the entire Mediterranean Region (Carafa et al. , 2015), and models of seismic hazard include geodetic data (Slejko et al., 2010; Cenni et al., 2015). To this purpose the 36 Seismic Zones ZS9 of Meletti et al. (2008) (http://zonesismiche. mi.ingv.it/App2.pdf ) are particularly useful, as a seismic catalogue is associated with each of them (Gruppo di Lavoro CPTI04, 2004). Consequently the Gutenberg Richter parameters of the statistical formula expressing the yearly number of events in the magnitude range (m, m dm) as a function of magnitude m in each ZS9 are available. Caporali et al. (2011) have shown how the statistical seismicity, maximum expected magnitude and geodetic shear strain rate can be used, in conjunction with the empirical formulas of Wells and Coppersmith (1994) for rupture area and average displacements, to set an upper limit to the average stress drop which is to be expected in a given province. The basic idea is to compute the strain rate associated to seismicity using the Gutenberg Richter parameters of that province, the empirical relations between magnitude m and Rupture Area (RA) and Average Displacement (AD) of Wells and Coppersmith (1996), and a maximum expected magnitude m max . In our case we identify a seismic province with each of the 36 Seismic Zones of Meletti et al. (2008), we assume the b value and maximum catalogue magnitude as in Gruppo di Lavoro CPTI04 (2004) and assume that the maximum expected magnitude as equal to the maximum catalogue magnitude 0.3. The a-value is computed for each seismic zone from the catalogue data, assuming the published b-value labeled b Co-04.4 (Caporali et al. , 2011). Imposing that the geodetic strain rate is not smaller than the seismic strain rate, computed with the Kostrov formula (Kostrov and Das, 1988) yields an upper limit on the static stress drop ∆σ of the seismic province: (5) where a WC a RA a AD and b WC b RA b AD (‘WC’ stands for ‘Wells and Coppersmith’), m is the shear modulus (assumed 30 GPa) and a s , b s are the Gutenberg Richter parameters of the seismic province . This estimate of the static stress drop can be made at any point of a seismic province, and can serve as term of comparison for the stress drop computed from strong motion spectra of individual earthquakes. Interface to the DISS 3.1.1. The output of the analysis is generated in tabular and pictorial form. Of particular interest are the kml files for use with Google Earth, as the output data can be visualized in conjunction with other layers, such as the DISS 3.1.1 of INGV (Basili et al. , 2008). We remark that the scale of the plotting symbols has been optimized for a close-up visualization. Figs. 1-3 provide a full scale visualization so that the symbols appear small. Fig. 1 shows an example of the ETRF2000 horizontal velocities of permanent GNSS sites in Italy and surrounding areas. For each site a GE ‘balloon’ is available, with all the relevant information. These include name, coordinates, velocities and standard deviations, principal strain rates (extension, compression, azimuth, shear and their uncertainties), the scale distance