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Authors: Lombardi, S.* 
Voltattorni, N.* 
Title: Rn, He and CO2 soil-gas geochemistry for the study of active and inactive faults
Issue Date: 2009
Keywords: seismotectonic investigations
soil-gas migration
Subject Classification04. Solid Earth::04.02. Exploration geophysics::04.02.01. Geochemical exploration 
Abstract: Two Italian areas, characterized by different seismological histories, were investigated in order to enhance the basic knowledge of gas migration mechanisms along fracture and fault surfaces during earthquakes. Sharp variations occur in the movement and concentration of some gaseous species due to the evolution of the local stress regime. The first area (named Colpasquale) is located in the central Italian region of Marche and provided a good location to study gas migration in a seismically active region. The area was devastated by a sequence of shallow earthquakes over a three month-long period (September-December, 1997). The occurrence of this catastrophic event as well as the long duration of the "seismic sequence", presented a unique opportunity to apply a study of gas migration to a zone undergoing active displacement. Soil-gas surveys were performed one day, one week, one year and two years after the main shock (Ms 5.6) in the Colpasquale area. In particular, results highlighted a change of the radon distribution during the three monitoring years indicating a variation of gas migration that may be linked to the evolution of the stress regime. The second study area is located in the Campidano Graben (southern part of Sardinia Island). This area is characterized by seismic quiescence, displaying an almost complete lack of historical earthquakes and instrumentally recorded seismicity. The consistently low values observed for all analysed gases suggest that the studied area is characterised by self-sealing probably non-active faults that prevent significant gas channelling. The comparison of data from both studied areas indicate that soil-gas geochemistry is useful to locate tectonic discontinuities even when they intersect non-cohesive clastic rocks (unconsolidated sediments) near the surface and thus are not visible (e.g. “blind faults”).
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