Statistical approach for the geochemical signature of two active normal faults in the western Corinth Gulf Rift (Greece) N. Voltattorni ⇑ , D. Cinti, L. Pizzino, A. Sciarra Istituto Nazionale di Geofisica e Vulcanologia (INGV), Via di Vigna Murata 605, 00143 Roma, Italy article info Article history: Available online 2 October 2014 Editorial handling by M. Kersten abstract Soil–gas measurements of different gas species were performed in two distinct areas of the Corinth Gulf Rift (Greece): the Aigion-Neos Erineos-Lambiri (ANEL) fault zone and the Rion-Patras fault zone. Both zones lie in one of the most seismically active areas of the Euro-Mediterranean region, where a fast-open- ing continental rift is located. In particular, the geochemical investigations were focused on fault seg- ments and fracture systems previously inferred by geomorphological, lithological and structural studies. In this work the applicability of soil–gas geochemistry surveys for the exploration of buried/hidden faults was tested by using various statistical methods. Moreover, a comprehensive geostatistical treat- ment of the collected data provided new insights into the control exerted by active structures on deep-seated gas migration towards the surface. In both investigated areas, the highest 222 Rn and CO 2 con- centration peaks correspond with zones where the interaction among fracture and fault segments was inferred by structural and morphological methods. This indicates a clear correlation between the shape and orientation of the anomalies and the different attitude and kinematic behavior of the faults recog- nized in the two areas. Furthermore, obtained results show that gases migrate preferentially through zones of brittle deformation by advective processes, as suggested by the relatively high rate of migration needed to obtain anomalies of short-lived 222 Rn in the soil pores. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Fault and fracture zones represent preferential migration path- ways, due to their enhanced permeability and porosity relative to surrounding rocks, along which gases can buoyantly migrate upwards. Accordingly, soil–gas surveys was recognized as a valid tool to trace faults and areas of brittle deformation especially when they have no clear expression at surface and/or when they are masked by unconsolidated lithologies (e.g. Ciotoli et al., 1998, 1999, 2003; Burton et al., 2004; Voltattorni and Lombardi, 2010; Neri et al., 2011). Identification of potentially active hidden faults is of significant relevance for both neotectonic studies and seismic hazard management (e.g. Bernard et al., 2006; Lewicki et al., 2003; Pizzino et al., 2004; Annunziatellis et al., 2008). In the present work a soil gas survey has been performed in two distinct areas of the Corinth Gulf Rift, in the Aigion-Neos Erineos- Lambiri (ANEL) fault zone and the Rion-Patras fault zone, respec- tively, where geometry of some fault segments and fracture systems have been inferred based on geomorphological and lithological discontinuities (Palyvos et al., 2005, 2007). As gas distribution in soil can display contrasting patterns even at a rela- tively small scale, the collection of numerous samples and the appropriate statistical processing of the collected data make the soil–gas method a powerful tool for geological and tectonic investi- gations, particularly in the detection of active faults and for the exploration of buried/hidden faults. 2. Origin and behavior of studied soil gas species Among soil gases, 222 Rn, CO 2 , 4 He, H 2 , CH 4 are the most reliable fault tracers (Toutain and Boubron, 1999). 222 Radon is a short-lived (3.8 days) radioactive gas produced from the 238 U decay chain. In diffusive systems it displays a poor mobility (Dubois et al., 1995) that, associated with its short half-life, limit its migration from the source rock. Along fault zones radon anomalies are produced by advective flow of gases that facilitate the transport of 222 Rn from greater depth into soils (e.g. Abdoh and Pilkington, 1989). Helium ( 4 He) is a rare gas produced from the 238 U decay chain and characterized by a high mobility in diffusive-advective sys- tems, a monatomic structure, chemical inertia and a low solubility in water (Jenkins and Cook, 1961). Helium has a low and constant concentration in air (5.220 ± 0.004 ppm; Holland and Emerson, 1990). Surface He anomalies are generally attributed to the upward http://dx.doi.org/10.1016/j.apgeochem.2014.09.011 0883-2927/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +39 06 51860607; fax: +39 06 51860507. E-mail address: nunzia.voltattorni@ingv.it (N. Voltattorni). Applied Geochemistry 51 (2014) 86–100 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem