Rn, He and CO2 soil gas geochemistry for the study of active and inactive faults

Two Italian areas, characterized by different seismological histories, were investigated to enhance the
basic knowledge of gas migration mechanisms 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 (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 3 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 study
gas migration in a zone undergoing active displacement. Soil gas surveys were performed 1 day, 1 week,
1 year and 2 years after the main shock (Ms 5.6) in the Colpasquale area. In particular, results highlight a
change in the Rn 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 analyzed gases suggest
that the studied area is likely characterized by sealed, non-active faults that prevent significant gas
migration. 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 near the surface
and thus are not visible (i.e., ‘‘blind faults”).

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