GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.1 9 whereas subhydrostatic pore pressures develop in areas affected by volumetric dilation. In the postseismic phase, the Δp values gradually dissipate because of fluid diffusion. After 20 days from the simulated earthquakes, the ongoing fluid diffusion in the medium (black arrows in Fig. 3i and j) gradually mitigates the postseismic pore pressures, which are approximately four times lower than the coseismic ones (Fig. 3g and h). Since pore pressures and stresses are coupled, the gradual dissipation of the coseismic Δp alters the stresses inside the medium, developing further deformations. Indeed, the modelled postseismic deformations two years after the simulated events show further subsidence of the hangingwall for the L’Aquila 2009 normal- fault earthquake (Fig. 3k) and uplift for the Emilia 2012 reverse-fault earthquake (Fig. 3l). Discussion and concluding remarks. The developed numerical model allowed to reproduce the interseismic, coseismic and postseismic phases for extensional and compressional earthquakes in Italy within a unified framework. The imposed geometrical and boundary conditions resulted in a distribution of stresses and strains in the earth’s crust that is compatible with the interseismic, coseismic and postseismic evolution of normal and reverse fault earthquakes. In detail, the simulation results show that the applied basal shear traction appears to be an essential factor to model the large-scale interseismic pattern since it allows for a first- order simulation of the ongoing crustal interseismic extension of the Central Apennines and compression of the Adriatic foreland and the north-eastern part of the Italian territory. The action of shear tractions and lithostatic forces generates a local concentration of stresses and strains in the presence of local heterogeneities or discontinuities, i.e., at the transition between the brittle locked fault and the ductile unlocked slipping fault during the interseismic stage (Fig. 3b and d). Such an interseismic strain partitioning provides maximum horizontal stress sufficient to exceed the friction on the locked brittle part of the fault, with the subsequent collapse of the hangingwall in case of extensional earthquakes (Fig. 3e) or the expulsion of the hangingwall in case of compressional earthquakes (Fig. 3f). The displacement profiles at zero depth in Fig. 3e and f are compared in with the observed displacements profiles form InSAR along the Line of sight of the satellite, taken along the sections A, and B Fig. 1a. The agreement between the modelled (continuous blue line) and measured (blue circles) LoS displacements (Fig. 3 m and n) is satisfactory for both normal and reverse-fault events. The instantaneous slip of the hangingwall perturbs the crustal pore fluid pressures, triggering groundwater flow in the postseismic phase from regions of higher pore pressures, which further compress, to regions of lower pore pressures, which further dilate. As a result, displacements gradually accumulate in the postseismic phase, according to the dissipation of pore pressure excess. The poroelastic compression/dilation of the medium causes the mainshock causative fault to further slip in the postseismic phase, which contributes to the accumulated ground displacements. Once the postseismic phase terminates, a new cycle of interseismic loading can start again. References Anderlini L., Serpelloni E., Belardinelli M.E.; 2016: Creep and locking of a low-angle normal fault: Insights from the Altotiberina fault in the Northern Apennines (Italy) . Geophys. Res. Lett. 43, 4321–4329. https://doi. org/10.1002/2016GL068604 Atzori S., Hunstad I., Chini M., Salvi S., Tolomei C., Bignami C., Stramondo S., Trasatti E., Antonioli A.,Boschi E.; 2009: Finite fault inversion of DInSAR coseismic displacement of the 2009 L’Aquila earthquake (central Italy) . Geophys. Res. Lett. 36, n/a-n/a. https://doi.org/10.1029/2009GL039293 Barba S., Carafa M.M.C., Boschi E.; 2008: Experimental evidence for mantle drag in the Mediterranean. Geophys. Res. Lett. 35, 1–6. https://doi.org/10.1029/2008GL033281 Carafa M.M.C., Barba S., Bird P.; 2015: Neotectonics and long-term seismicity in Europe and the Mediterranean region. J. Geophys. Res. Solid Earth 120, 5311–5342. https://doi.org/10.1002/2014JB011751 Carminati E., Vadacca L.; 2010: Two- and three-dimensional numerical simulations of the stress field at the thrust front of the Northern Apennines, Italy. J. Geophys. Res. 115, B12425. https://doi.org/10.1029/2010JB007870 Doglioni C., Barba S., Carminati E., Riguzzi F.; 2011: Role of the brittle-ductile transition on fault activation . Phys. Earth Planet. Inter. 184, 160–171. https://doi.org/10.1016/j.pepi.2010.11.005