Abstract
Soil surveys were performed in Medolla (Italy), a peculiar area characterized by spotty high soil temperature, gas vent, and lack of vegetation, to determine the migration mechanisms and spatial behavior of gas species. Hereby we present soil gas measurements and their isotopic ratios measured between 2008 and 2015, including the 2012 Emilia-Romagna seismic sequence. We found that soil gas concentrations markedly changed during the main shocks of May 20 and 29, 2012 (Mw 6.1 and 6.0, respectively), highlighting the presence of a buried fault intersecting the gas vents. We suggest that crustal dilation associated with seismic activity favored the uprising of geogas towards the surface. Changes in the isotopic signature highlight the contribution of two distinct sources, one deeper, thermogenic and another superficial related to organic-rich layer, whose relative contribution varied before, during and after the earthquake. We suppose an increase of microbial component likely due to the ground shaking of shallower layers linked to seismic sequence, which masks the thermogenic contribution. Although the changes we detect are specific for an alluvial plain, we deduce that analogous processes may be active elsewhere, and that soil gas geochemistry represents an useful tool to discriminate the gas migration related to seismic activity.
Highlights
Soil gas geochemistry in seismically active areas has been widely used to localize buried faults and co-seismic fractures[1,2,3,4,5]
Hereby we present 37 soil gas concentrations (CO2, CH4, H2, He, Ne, C2H6, 222Rn) together with the isotopic ratios of CH4 and CO2 (δ13C-CH4, δ13C-CO2, δD-CH4) that were periodically measured in the period 2008–2015 and immediately after the 2012 Emilia seismic sequences
The lack of an evolutionary trend between thermogenic and mixed-microbial origin suggest that a fractioning process does not occur
Summary
Soil gas geochemistry in seismically active areas has been widely used to localize buried faults and co-seismic fractures[1,2,3,4,5]. Measurements were performed near Medolla (a small municipality in the Emilia-Romagna Region, Italy; Fig. 1a), in a farming area less than 1 km[2] wide, where the presence of a remarkably high temperature of the soils (up to 48.5 °C) associated with methane seepage, has been known since 18938. In some cases, this seepage phenomenon is highlighted by the occurrence of several subcircular areas (
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