GNGTS 2013 - Atti del 32° Convegno Nazionale

algorithm, while constraining the slip-rate of each fault patch to be equal or less than the long-term slip-rate estimated from the uniform-slip block model (see Tab. 1). This approach allows us to highlight portions of the fault surface that are characterized by low coupling (i.e., creeping patches) or elastic coupling (i.e. elastic slip-deficit). However, in this way the number of model parameters is greater than the number of data, and we perform a regularization of the inversion adding a smoothing constraint to the solution. The regularization is weighted by a factor β which controls the relative importance of minimizing the reduced chi-squared versus minimizing the roughness of the slip. We choose the optimal value of β equal to 0.7, following a trade-off curve approach (Harris and Segall, 1987). We applied a further constraint to the slip-deficit, forcing it to tape to zero at the bottom edge, at depth of ~13 km, a depth roughly corresponding to the brittle-ductile transition, expected for depth below of 11 km (Boncio et al. , 2004). The final slip-deficit distribution gives a total (the whole model) reduced chi-squared value that is close to that obtained in the uniform slip inversion, but the reduced chi-squared statistics, computed on local stations, drops to 5.27. We represent the slip-deficit distribution as Interseismic Coupling (IC), defined as the ratio between slip-deficit on each patch and the long-term velocity slip-rate (considered in this study -1.5 mm/yr from Tab. 1). The IC ranges between 0 and 1, where 0 means fully uncoupled fault patches (i.e. aseismic creeping) and 1 means fully coupled fault patches (i.e. elastic asperities). Fig. 3 shows the final IC distribution, which shows two main asperities on northern part of the curved surface, and the relocated microseismicity recorded between October 2000 and May 2001 from Chiaraluce et al. (2007), selected within +-1.5 km from the ATF surface. The IC distribution shows a correlation between the selected microseismicity and a narrow uncoupled area, located between the two asperities, which position corresponds exactly of bottom edge of Gubbio fault. We perform a resolution test, adopting a checkerboard approach, in order to evaluate the reliability of our IC distribution. These tests show that the transition zone between two asperities is resolved by our data. Conclusions. Using a self-consistent kinematic block modeling we study the northern sector of the Umbria-Marche Apennines, where several GPS stations show SW-NE oriented extensional deformation. We tested different block model geometries in order to understand which fault system is accommodating the tectonic extension. We found that the best model is the one considering two fault systems, i.e. the Alto Tiberina LANF and the antithetic high-an- gle Gubbio normal fault, since we obtain lower residuals on data and kinematic agreement with geological slip-rates (Collettini et al. , 2003; Pucci et al. , 2003). Nevertheless obtaining systematic residuals at a group of GPS sites located between the two fault systems, we param- eterized the ATF fault as a, more realistic, curved surface to infer the distribution of interseis- mic coupling, which is validated by numerous resolution tests. The obtained IC distribution shows a correlation between relocated microseismicity and uncoupled patches attributed to aseismic creeping behavior (Vergne et al. , 2001; Schmidt et al. , 2005; Rolandone et al. , 2008), which could be explained by the presence of fluid overpressure, as was hypothesized by Collet- tini (2002). Otherwise this correlation has been verified with a very small quantity of events (almost 400) and it might be of interest to evaluate this correlation with future available data. References Altamimi, Z., X. Collilieux, and L. Métivier (2011), ITRF2008: an improved solution of the international terrestrial reference frame, J Geodesy , 85(8), 457-473, doi:10.1007/s00190-011-0444-4. Boncio, P., & Lavecchia, G. (2000). A structural model for active extension in Central Italy. Journal of Geodynamics, 29, 233-244. Bennett, R. A. et al. (2012), Syn-convergent extension observed using the RETREAT GPS network, northern Apennines, Italy, J Geophys Res , 117 (B4), doi:10.1029/2011JB008744. Blewitt, G., and D. Lavallée (2002), Effect of annual signals on geodetic velocity, J. Geophys. Res. , 107(B7), 2145, doi:10.1029/2001JB000570. 152 GNGTS 2013 S essione 1.2