'EARTHQUAKE RESERVOIRS' AND NON-VOLCANIC DEGASSING IN AN ACTIVELY EXTENDING OROGEN, SOUTHERN APENNINES, ITALY

The southern Apennines fold and thrust belt has been undergoing post-orogenic extension since ca. 400 kyr. Effective decoupling between deep and shallow structural levels is related to the strong rheological contrast produced by a fluid-saturated, clay-rich mélange zone interposed between buried autochthonous carbonates – continuous with those exposed in the Apulian foreland – and the allochthonous units. This mélange zone also acts as a seal preventing the migration of deep-seated aqueous fluids – as well as oil in the Basilicata region, which hosts the largest Europe’s onshore oil fields – towards the surface. The presence of water reservoirs below the mélange zone is shown by a high V_P/V_S ratio – which is a proxy for densely fractured, fluid-saturated crustal volumes – recorded by seismic tomography within the buried autochthonous carbonates and the top part of the underlying basement. These crustal volumes, in which background seismicity is remarkably concentrated – hence the term ‘earthquake reservoir’ – are fed by fluids rising along the major active faults and are laterally bounded by the faults themselves, which typically act as barriers for cross-fault fluid flow and as conduits for along-fault flow due to the high permeability of their damage zones. High pore fluid pressures, decreasing the yield stress, are recorded by low stress-drop values associated with the earthquakes. On the other hand, the mountain belt is characterized by substantial gas flow to the surface, recorded as both distributed soil gas emissions and vigorous gas vents. We measured a CO₂ flux up to 34000 g/m^-2 per day at a gas vent, as well as large amounts of He (up to 52 ppm), Rn (up to 228 kBq/m³) and CH₄ (up to 5000 ppm). Overpressured CO₂ has been proposed as triggering normal fault earthquakes in the Apennines. However the continuous, huge soil gas emission suggests that CO₂, although locally accumulated in gas caps at the top of buried carbonate culminations and sealed by the mélange, is not effectively trapped at deeper seismogenic levels, which are instead saturated with aqueous fluids. Fault lubrication processes are driven by water pressurization along mature, inherited and multiply reactivated fault zones, producing a substantial decrease of dynamic friction marked by relatively low mean seismic radiation efficiency.