Abstract
<p>Following the earthquake (M<sub>L</sub>=6.0) of 24 August 2016 that affected large part of the central Apennine between the municipalities of Norcia (PG) and Amatrice (RI) (central Italy), two soil gas profiles (i.e., <sup>222</sup>Rn, <sup>220</sup>Rn, CO<sub>2</sub> and CO<sub>2</sub> flux) were carried out across buried and exposed coseismic fault rupture of the Mt. Vettore fault during the seismic sequence. The objective of the survey was to explore the mechanisms of migration and the spatial behaviour of different gas species near still-degassing active fault. Results provide higher gas and CO<sub>2</sub> flux values (about twice for <sup>222</sup>Rn and CO<sub>2</sub> flux) in correspondence of the buried sector of the fault than those measured across the exposed coseismic rupture. Anomalous peaks due to advective migration are clearly visible on both side of the buried fault (profile 1), whereas the lower soil gas concentrations measured across the exposed coseimic rupture (profile 2) are mainly caused by shallow and still acting diffusive degassing associated to faulting during the seismic sequence. These results confirm the usefulness of the soil gas survey to spatially recognise the shallow geometry of hidden faults, and to discriminate the geochemical migration mechanisms occurring at buried and exposed faults related to seismic activity.</p>
Highlights
Local increases in radon emanation along faults could be caused by a number of processes, including precipitation of parent nuclides caused by local radium content in the soil [Tanner, 1964; Zunic et al, 2007], increase of the exposed area of faulted material by grainsize reduction [Holub and Brady, 1981; Koike et al, 2009; Mollo et al, 2011], and carrier gas flow around and within fault zones [e.g., King et al, 1996; Annunziatellis et al, 2008]
Given the acceptance that the concentrations of radon gas are the result of advective gas flow associated with elevated permeability in fault zones, soil gas surveys are widely regarded to be an effective tool to map buried or blind faults not detected during mapping of the surface geology [King et al, 1996; Burton et al, 2004; Ciotoli et al, 2007, 2015, 2016]
CO2 flux values show a higher mean for the P1, approximately twice than that calculated for the P2, and a high variability being affected by the vegetation, shallow soil characteristics, as well as meteorological conditions [Metzger et al, 2008]
Summary
Soil gas survey has been widely used to trace buried faults and to study the behaviour in the shallow environment of endogenous gases with different origins (i.e., trace gases, i.e., radon and helium, and carrier gases i.e., carbon dioxide, nitrogen, methane, etc.) and [e.g., King, 1985; Baubron et al, 2002; Ciotoli et al, 2007; Fu et al, 2008; Walia et al., 2009; Ciotoli et al, 2014; Bigi et al, 2014; Sciarra et al, 2014]. Over the past several years’ soil gases has captured considerable attention as earthquake precursors [Wakita et al, 1980; Reddy et al, 2004; Walia et al, 2009; Perez et al, 2007; Ghosh et al, 2009; Hashemi et al, 2013; Petraki et al, 2015], that the stress/strain changes related to seismic activity may force crustal fluid to migrate up, especially along active faults, thereby altering the geochemical characteristics of the fault zone at surface [Rice, 1992; Sibson, 2000; Collettini et al, 2008] The migration of these gases by diffusion and/or advection along buried active faults can generate shallow anomalies with concentrations significantly higher than background levels; these anomalies can provide reliable information about the location and the geometry of the shallow fracturing zone, as well as about the permeability within the fault zone [King et al, 1996; Baubron et al, 2002; Ciotoli et al, 2007; Annunziatellis et al, 2008; Bigi et al, 2014; Sciarra et al, 2014]. In order to explore the possible mechanisms of soil gas transport near active fault, we infer that gas anomalies are originated by both (1) advective flow along the fault zone, and (2) increased diffusive migration in the soil due to surface processes associated with faulting
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