GNGTS 2013 - Atti del 32° Convegno Nazionale

data, solves for six parameters: the two horizontal velocity components and the four velocity gradients (Feigl et al. , 1990). The weights are estimated by scaling the variance associated with the velocity data with an exponential scaling function e (djl/D) , where d jl is the distance between the j th node of the grid and the l th GPS station, and D is the distance decay factor. The GPS sites lying at a distance lower than D from the j th point of the grid give a significant contribution to the estimate of the interpolated velocity and of the strain rate (Shen et al. , 1996). In order to improve the reliability of results, we have adopted two geometric criteria: the interpolated values are taken as acceptable only when at least three GPS sites are located at a distance lower than D from the point considered and the sites considered must be uniformly distributed in the surrounding region (one in each 120° angular sector). The resulting strain rate field obtained, by using a D value (50 km) about three times larger than the average spacing of the network (Fig. 2), points out the presence of fairly coherent regimes in the various sectors of the zone considered. A compressional regime, with SSE–NNW shortening axis, dominates in the East- ern Southern Alps, at the northern collisional border of the Adriatic plate with the Eurasian domain. A roughly N–S compression, associated with minor orthogonal lengthening occurs in the Po Valley area which overlies the outer buried thrusts of the Northern Apennine belt. This regime is consistent with the strain field that has been associated with the recent strong earthquakes that occurred in the Emilia zone since May 20, 2012 (Pondrelli et al. , 2012). A dominant extensional regime, with SW–NE lengthening axis, coherently occurs in the North- ern Apennines, in agreement with the features of the extensional tectonics that has formed the main troughs running along the axial part of the chain (e.g., Boncio and Lavecchia, 2000). A transtensional regime mainly occurs in the central Apennines, where major sinistral fault systems are recognized (e.g., Piccardi et al. , 2006). Another parameter that can be derived from the GPS displacement field is the total strain rate ( TSR ), which is defined by the following relation where , and are the horizontal components of the strain rate tensor (Kreemer et al. , 2003). The pattern of TSR on a grid of 0.1° x 0.1° is shown in Fig. 3. The comparison of this pattern with the distribution of the earthquakes of M ≥ 4 occurred in the same area since 01/01/2001, shows that most seismicity is located where the TSR values are relatively high. Fig. 3 – Second invariant of 2D strain rate tensor (Total strain rate = TSR) estimated from the velocity field (Fig. 1). Circles indicate the locations of the earthquakes occurred since 01/01/2001, with magnitude M ≥4; and depth greater than 40 km (ISIDe) 32 GNGTS 2013 S essione 2.1