GNGTS 2018 - 37° Convegno Nazionale

164 GNGTS 2018 S essione 1.1 CRUSTAL DEFORMATION AND SEISMICITY MODULATED BY GROUNDWATER RECHARGE OF KARST AQUIFERS: INSIGHTS FROM THE IRPINIA FAULT (ITALY) N. D’Agostino 1 , F. Silverii 2 , O. Amoroso 3 , V. Convertito 4 , F. Fiorillo 5 , G. Ventafridda 6 , A. Zollo 7 1 Istituto Nazionale Geofisica e Vulcanologia, Roma, Italy 2 Scripps Institution of Oceanography (SIO), University of California, San Diego, USA 3 Department of Physics E.R. Caianiello, University of Salerno, Fisciano (SA), Italy 4 Istituto Nazionale Geofisica e Vulcanologia, Napoli, Italy 5 Dipartimento di Scienze e Tecnologie (DST), University of Sannio, Benevento, Italy 6 Approvvigionamento Idrico (DIRAP), Acquedotto Pugliese S.p.A., Bari, Italy 7 Department of Physics E. Pancini, University of Naples Federico II, Napoli, Italy Variations of the spatial and temporal distribution of hydrological masses deform the lithosphere (van Dam et al. , 2001) and modulate stress buildup in active fault zones (Ader et al. , 2014). One of the most obvious manifestations of hydrological forcing is expressed by modulations of seismicity (Heki, 2003; Saar and Manga, 2003) triggered by stress variations as small as a few kPa (Christiansen et al. , 2007). Due to the capacity of channeling large amount of water in the conduits network, karst aquifers rapidly develop large increases in hydraulic head after rainfall increasing the hydraulic head loading upon the fluid saturated, poroelastic crust. This property has been invoked to explain rainfall-triggered seismicity (Miller, 2008) assuming the existence of a hydraulically connected fracture system and downward diffusion of surface pore pressures. A statistically significant causal relationship has been proposed based on fluid diffusion and rate-and-state-dependent frictional nucleation (Hainzl et al. , 2006). However, it is unclear if seismicity is only due to pore pressure diffusion or may also be triggered in response to poroelastic solid stress changes (Segall, 1989) overcoming the shortcomings associated with the requirement of an hydraulically-connected fracture system to seismogenetic depths. Along the Mediterranean-Alpine belt, large parts of tectonically-active countries (Italy, Balkans, Turkey) are underlain by carbonate rocks where hydrological forcing from groundwater recharge in karst aquifers provides the opportunity to discriminate alternative models and investigate the modulation of stress buildup and seismicity on active fault systems. Here we comprehensively investigate the mechanism relating groundwater storage, stress buildup and seismicity in the Southern Apennines (Italy) exploiting the concurrent presence of karst regions with dense seismic and geodetic networks building upon our previous work (Silverii et al. , 2016, 2019). We use GPS (Global Positioning System), hydrological and seismicity data to show that poroelastic strain in the shallow crust (0-3.5 km) controls seasonal and multiannual modulation of seismicity along the Irpinia Fault Zone (IFZ, Southern Italy) without requiring a hydraulically connected fracture system from the surface to hypocentral depths. We suggest that groundwater recharge of karst aquifers along the IFZ produce stress perturbations large enough to modulate strain accumulation and seismicity and temporarily modify the probability of nucleation of seismic events such as the 1980 Irpinia, MS 6.9, earthquake. References Ader, T., Lapusta, N., Avouac, J., & Ampuero, J. (2014). Response of rate-and- state seismogenic faults to harmonic shear-stress perturbations. Geophys. J. Int., 198(1), 385-413. doi: 10.1093/gji/ggu144. Christiansen, L., Hurwitz, S., & Ingebritsen, S. (2007). Annual modulation of seis- micity along the san andreas fault near parkfield, ca. Geophys. Res. Lett., 34(L04306). doi: 10.1029/2006GL028634. Hainzl, S., Kraft, T., Wassermann, J., Igel, H., & Schmedes, E. (2006). Evidence for rainfall-triggered earthquake activity. Geophysical Research Letters, 33(19). doi: 10.1029/2006GL027642. Heki, K. (2003). Snow load and seasonal variation of earthquake occurrence in japan. Earth and Planetary Science Letters, 207(1), 159–164. doi: 10.1016/ S0012-821X(02)01148-2. Miller, S. A. (2008). Note on rain-triggered earthquakes and their dependence on karst geology. Geophysical Journal International, 173(1), 334-338. doi: 10.1111/j.1365-246X.2008.03735.x. Saar, M., & Manga, M. (2003). Seismicity induced by seasonal groundwater recharge at mt. hood, oregon. Earth and Planetary Science Letters, 214(3), 605 - 618. doi: 10.1016/S0012-821X(03)00418-7.