GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 1.1 45 confined high V P /V S and low V P ×V S volume, co-located with the present-time background seismicity (Fig. 1b-c). The Q P model reaches the highest values in the volume containing the Irpinia 1980 event hypocentre between 8-12 kmdepths (Fig. 3a). The Q S model also shows strong lateral variations along a SW-NE direction with a major variation occurring in correspondence with the 1980 earthquake rupture (Fig. 3b). In the volume in which Q P reaches highest values, at about 10 km depth, we constrain the porosity to be in the range of 4-5% and the consolidation parameter to be around 5-9, with the possible fluid mixing being both brine-CO 2 and CH 4 -CO 2 . This consolidation parameter range indicates high pore pressures at these depths (Fig. 3c). We aspect that the 4D tomographic images will shows spatial and temporal variability of elastic properties of host medium. This variability can be correlated to the fluid saturations changes and migration in the different time epochs throughout the propagation medium. Discussion. The presence of liquid and gas fluid phases in a fault volume and the inferred high pore pressure values have important consequences on seismicity generation. In fact, the presence of fluids inside the fault gauge may enhance seismicity due to lubrication mechanisms and by an increase of pore pressure in the medium embedding the faults. In the IFZ, the modelling of micro-earthquake spectra has provided a rather low average seismic radiation efficiency (Zollo et al. , 2014), thus implying that the rupture lubrication mechanisms are not favoured. Therefore, we suggest that the dominant mechanism triggering the micro-seismicity at the IFZ is the pore pressure increment, induced by fluid diffusion in the host rock medium (Dvorkin et al. , 2000). When rocks are close to a critical state of failure, a perturbation of the pore pressure, modifying the effective normal stress, can lead to the occurrence of a seismic fracture (Nur and Booker, 1972). In particular, at the considered depths, gasses more than liquids may significantly increase the pore pressure up to a level for which it equals the lithostatic pressure (Hantschel and Kauerauf, 2009). The results of the up-scaling procedure, especially in terms of consolidation parameter, allow us to interpret the investigated volume at 8–10 km depth as highly fractured and liquid-gas saturated, where the high pore pressure is directly responsible for the seismicity triggering mechanism and where, in fact, most of seismicity occurs. References Amoroso O., Ascione A., Mazzoli S., Virieux J. and Zollo A.; 2014: S eismic imaging of a fluid storage in the actively extending Apennine mountain belt, southern Italy . Geophys. Res. Lett., 41 , 3802-3809, doi: 10.1002/2014GL060070. Amoroso O., Russo G., De Landro G., Zollo A., Garambois S., Mazzoli S., Parente M. and Virieux J.; 2017: From velocity and attenuation tomography to rock physical modeling: Inferences on fluid-driven earthquake processes at the Irpinia fault system in southern Italy. Geophys. Res. Lett., 44 , 6752-6760, doi:10.1002/2016GL072346. Amoroso O., Festa G., Bruno P.P., D’Auria L., De Landro G., Di Fiore, V., Gammaldi S., Maraio S., Pilz M., Roux P. and Russo G.; 2018: Integrated tomographic methods for seismic imaging and monitoring of volcanic caldera structures and geothermal areas. Journal of Applied Geophysics, 156 , 16-30. Ascione A., Mazzoli S., Petrosino P. and Valente E.; 2013: A decoupled kinematic model for active normal faults: Insights from the 1980, M S 6.9 Irpinia earthquake, southern Italy. Geol. Soc. Am. Bull., 125 , 1239–1259, doi:10.1130/B30814.1. De Landro G., Amoroso O., Stabile T.A., Matrullo E., Lomax A. and Zollo A.; 2015: High precision Differential Earthquake Location in 3D models: Evidence for a rheological barrier controlling the micro-seismicity at the Irpinia fault zone in southern Apennines . Geophy. J. Internat., 203 , 1821-1831, doi:10.1093/gji/ggv397. Dupuy B., Asnaashari A., Brossier R., Garambois S., Métivier L., Ribodetti A. and Virieux J.; 2016: A downscaling strategy from FWI to microscale reservoir properties from high-resolution images. Lead. Edge, 35 (2), 146–150, doi:10.1190/tle35020146.1. Dvorkin J., Helgerud M. B., Waite W. F., Kirby S. H. and Nur A.; 2000: Introduction to physical properties and elasticity models, in Natural Gas Hydrate , 245–260, Springer, Netherlands. Hantschel T. and Kauerauf A. I.; 2009: Fundamentals of Basin and Petroleum Systems Modeling. XVI, 476, Springer, Berlin, doi:10.1007/978-3-540-72318-9. Hardebeck L. and Hauksson E.; 1999: Role of fluids in faulting inferred from stress field signature . Science, 285 , 236–239.