GNGTS 2016 - Atti del 35° Convegno Nazionale

230 GNGTS 2016 S essione 1.2 A preliminary evaluation of the forecast moment tensor solutions in Italy P. Roselli, M.T. Mariucci Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy We present some Italian present-day stress field parameters in a contest of Probabilistic Seismic Hazard Analysis (PSHA) intended as a combination of seismicity rate models, Ground Motion Prediction Equations (GMPEs) and stress field. The use of GMPEs represents the main source of uncertainty in PSHA. The reduction of this epistemic uncertainty is closely related to the expected focal mechanism type associated to the next large earthquake at the given site (Convertito and Herrero, 2004; Strasser et al. , 2006; Roselli et al. , 2016). Then, to reduce this uncertainty, GMPEs can be more accurate if the expected focal mechanism is known in advance. In this framework, we present the application of the procedure named Total Weighted Moment Tensor (Kostrov, 1974; Selva and Marzocchi, 2004; Roselli et al. , 2015, 2016) providing the probability to observe in the future a normal (NF), reverse (RF) or strike-slip (SS) faulting event and the average distribution of the P, T and N axes, for each spatial cell considered (0.1° x 0.1° regular spatial grid) for the whole Italian territory. We estimate the seismic moment tensor parameters collecting past-data present in a maximum distance of 50 km from the centre of each cell. Then, we sum and mediate the contributions by weighing them according to inverse of squared distances. From the diagonalization of the mean moment tensor matrix obtained, we extract the P, T and N axes. The last step of our analysis consists in the establishment of a conditional probability of the focal mechanism type (for each cell) that satisfies the Kolmogorov axioms and can be interpreted in the Bayesian perspective of bet quotient (Roselli et al. , 2015, 2016). Then, we extract information such as prevalent moment tensor solution, expected focal mechanism type (NF, RF or SS) with the associated conditional probability. Furthermore, we extract the related maximum horizontal stress (SH max ) orientation by using the Zoback (1992) relations assuming the stress regime associated. We compare the obtained forecasting results with some earthquakes recently occurred in Italy that have not been used to calibrate the model. The data involved for the forecasting computation (392 well constrained events with Mw ≥ 4 and depth ≤ 40 km), from 1908 to 2015 belong to a significant focal mechanism catalogue (European- Mediterranean Regional Centroid Moment Tensor database, http://www.bo.ingv.it/RCMT/ ; and “The Italian CMT dataset from 1976 to the present”, http://www.bo.ingv.it/RCMT/ , Pondrelli et al., 2006) and are included in the latest present-day stress field data release for Italy (Montone and Mariucci, 2016). The last large earthquake data used for the comparison are from Istituto Nazionale di Geofisica e Vulcanologia (INGV; http://cnt.rm.ingv.it/) . In particular, we use the major events of the ongoing Amatrice seismic sequence (Mw 6.0), Campobasso (Umbria, Mw 4.3), Terni (Umbria-Marche zone, Mw 4.1), Ionian See (Mw 4) and Siracusa (Sicily, Mw 4.2) The achieved results will be used in the next PSHAmodel for Italy under construction. References Convertito V. and Herrero, A.; 2004. Influence of Focal Mechanism in Probabilistic Seismic Hazard Analysis , Bull. Seismol. Soc. Am., 94(6), 2124-2136. Kostrov, B.V.; 1974. Seismic moment and energy of earthquakes, and seismic flow of rock , Izv. Acad. Sci. USSR, Phys. Solid Earth, 1, 23-44. Montone, P. and Mariucci, M. T.; 2016. The new release of the Italian contemporary stress map , Geophys. J. Int., 205, 1525–1531, doi: 10.1093/gji/ggw100. Pondrelli, S., Salimbeni, S., Ekström, G., Morelli, A., Gasperini, P. and Vannucci, G.; 2006. The Italian CMT dataset from 1977 to the present , Phys. Earth Planet. Int., 159(3-4), 286-303, doi:10.1016/j.pepi.2006.07.008. Roselli, P., Marzocchi, Montone, P. and Mariucci, M.T.; 2015. Earthquake Focal Mechanism Forecasts and Applications in the PSHA in Italy . American Geophysical Union, Fall Meeting 2015 , abstract S14B-04. Roselli, P., Marzocchi W. and Faenza, L.; 2016. Toward a new probabilistic framework to score and merge Ground- Motion Prediction Equations: the case of the Italian region . Bull. Seismol. Soc. Am., 106(2), 720-733, doi: 10.1785/0120150057.