Structural controls on a carbon dioxide-driven mud volcano field in the Northern Apennines (Pieve Santo Stefano, Italy): Relations with pre-existing steep discontinuities and seismicity.

The strong release of carbon dioxide (CO2)-rich fluids in the axial zone of the Northern Apennines, near Pieve Santo Stefano, is analysed in relation to both structural controls and the microseismicity that followed the earthquake of 26 November 2001. Some vents recorded a transition from gas-dominated emission to post-seismic mud extrusion. The co-seismic rupture of a portion of the NW-trending Alto Tiberina Fault, which bounds the Quaternary Upper Tiber Basin, presumably broke the hydraulic seal trapping overpressured CO2-rich fluid compartments and released a fluid pressure pulse. Fault pattern analysis has revealed that venting localises along NE-trending steep faults associated with the regional transverse Arbia-Val Marecchia Line bounding the basin to the northwest. It is proposed that the transverse faults have been acting as permeable pathways allowing the post-seismic transport of overpressured fluids to the surface, during which the mud was mobilised. A hypothetical correlation is also established between failure mechanisms and the 2001 aftershocks. Results of Mohr constructions indicate that frictional fault reactivation is a suitable mechanism for explaining the seismic events along the NW-trending normal fault, while hydraulic fracturing is the most suitable to describe the seismicity occurring along the transverse faults.