HYDROLOGICAL PROCESSES Hydrol. Process. 25, 1754–1764 (2011) Published online 28 December 2010 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.7933 Impact of the 6 April 2009 L’Aquila earthquake on groundwater flow in the Gran Sasso carbonate aquifer, Central Italy Antonella Amoruso, 1 Luca Crescentini, 1 Marco Petitta, 2 * Sergio Rusi 3 and Marco Tallini 4 1 Dipartimento di Fisica, Universit` a di Salerno, Via Ponte Don Melillo, 84084 Fisciano, Salerno, Italy 2 Dipartimento di Scienze della Terra, Universit` a di Roma ‘La Sapienza’, P.le Aldo Moro 5, 00185 Roma, Italy 3 Dipartimento di Geotecnologie per l’Ambiente e il Territorio, Universit` a di Chieti-Pescara ‘G. D’Annunzio’, Via dei Vestini 31, 66013 Chieti, Italy 4 Dipartimento di Ingegneria delle Strutture, delle Acque e del Terreno, Universit` a dell’Aquila, Nucleo Industriale di Bazzano, 67100 Monticchio, L’Aquila, Italy Abstract: The Mw D 6Ð3 L’Aquila earthquake on 6 April 2009 produced a mainshock that caused significant changes in the hydrogeology of the Gran Sasso carbonate fractured aquifer: (i) the sudden disappearance at the time of the mainshock of some springs located exactly along the surface trace of the Paganica normal fault (PF); (ii) an immediate increase in the discharge of the Gran Sasso highway tunnel drainages and of other springs and (iii) a progressive increase of the water table elevation at the boundary of the Gran Sasso aquifer during the following months. Using the data collected since the 1990s that include aftershock monitoring as well as data regarding spring discharge, water table elevations, turbidity and rainfall events, a conceptual model of the earthquake’s consequences on the Gran Sasso aquifer is proposed herein. In this model that excludes the contribution of seasonal recharge, the short-term hydrologic effects registered immediately after the mainshock are determined to have been caused by a pore pressure increase related to aquifer deformation. Mid-term effects observed in the months following the mainshock suggest that there was a change in groundwater hydrodynamics. Supplementary groundwater that flows towards aquifer boundaries and springs in discharge areas reflects a possible increase in hydraulic conductivity in the recharge area, nearby the earthquake fault zone. This increase can be attributed to fracture clearing and/or dilatancy. Simulations by numerical modelling, related to pore pressure and permeability changes with time, show results in accordance with observed field data, supporting the conceptual model and confirming the processes that influenced the answer of the Gran Sasso aquifer to the L’Aquila earthquake. Copyright  2010 John Wiley & Sons, Ltd. KEY WORDS earthquake; groundwater flow; Central Italy; modelling Received 22 April 2010; Accepted 11 October 2010 INTRODUCTION The Mw D 6Ð3 earthquake that struck the L’Aquila region (Central Apennines) in Central Italy on 6 April 2009 (01 : 32 GMT) occurred at a depth of approxi- mately 9 km on the NW–SE trending Paganica normal fault (PF) (Bagnaia et al., 1992; Boncio et al., 2004). The earthquake resulted in coseismic surface rupturing faults (Chiarabba et al., 2009; Walters et al., 2009; Bon- cio et al., 2010). Envisat and COSMO-SkyMed DInSAR interferograms (Atzori et al., 2009) as well as GPS data (Anzidei et al., 2009) of the earthquake were initially analysed. By taking into account the strike direction of the PF according to focal mechanism estimates, the best fit solution for the mainshock was represented by an extensional fault that is approximately 16-km long and 12- to 13-km wide and that dips 47–57 ° south- west with a maximum slip of approximately 90 cm. This interpretation agrees with seismological data and the observed coseismic surface ruptures. Aseismic slip * Correspondence to: Marco Petitta, Dipartimento di Scienze della Terra, Universit` a di Roma ‘La Sapienza’, P.le Aldo Moro 5, 00185 Roma, Italy. E-mail: marco.petitta@uniroma1.it occurred on the same fault in the first hours after the event; its seismic moment was about 10% of the main shock seismic moment (Amoruso and Crescentini, 2009). The PF is a fault segment that is part of a regional network of NW–SE trending normal faults in the Cen- tral Apennines. The Apenninic thrust belt is part of the accretionary wedge caused by the roll back of the Adri- atic subduction towards the east (Doglioni et al., 1998). The Quaternary–Neogene normal faults, caused by the subsequent west-to-east migration of the regional exten- sional regime, govern the intra-montane basin evolu- tion and its filling through continental clastic deposits such as the L’Aquila Plain and Campo Imperatore (Cav- inato and De Celles, 1999). The impact of earthquakes on aquifers is well known (Nur, 1974; Wakita, 1975; Rojstaczer et al., 1995; Roeloffs, 1998; Tokunaga, 1999; Manga, 2001; Manga and Wang, 2007). Commonly, studies focus on both the liquefaction coseismic events (Holzer et al., 1989; Galli, 2000) and stream-aquifer changes in drainage of groundwater in aquifers dominated by inter-granular porosity (Basler et al., 1994; Sato et al., 2000; Montgomery and Manga, 2003; Montgomery et al., 2003; Wang et al., 2004). The consequences of seismicity Copyright  2010 John Wiley & Sons, Ltd.