Long-term (1997-2007) geochemical monitoring of gases from the Umbria-Marche region

The paper presents the results of ten-year long geochemical monitoring carried out in the area hit by the 1997–98 Umbria–Marche seismic sequence (Northern Apennines, Italy) showing the advancement in our understanding of the reciprocal relationships between fluids and tectonic structures in this area. Long-term geochemical monitoring has allowed us to define the geochemical features of the gas phase and to model its origins and interactions. All the released gases, including dissolved and venting gases, have predominantly crustal origin and have suffered from substantial modifications in their geochemical features. The whole seismogenic process is considered to be responsible for the modifications recorded over a wide time interval, including the production of CO2 from mechanical stress on carbonate rocks. We interpret the modifications occurred to the gas phase during the 1997–98 period as related to faulting activity, while those recorded after the end of the seismic crisis are interpreted as being a consequence of the crustal relaxation. The data show that it is almost impossible to interpret the temporal variation without coupling long-term geochemical monitoring and knowledge of the genetic features of the fluids. Therefore the information coming from the circulating fluids is a powerful tool to better understand the physical processes governing the faulting activity.