Fluid Overpressure and Earthquake Triggering in Faulted Evaporitic Sequences

The mainshocks of the 1997 Umbria-Marche seismic sequence (M above 5) nucleated at about 6 km depth, within the Triassic Evaporites (TE), made of interbedded anhydrites and dolostones. Previous studies suggest that the time-space evolution of the aftershock sequence was driven by the coseismic release of high-pressure fluids trapped within the TE. In order to understand whether CO2 fluid overpressure can be maintained up to the coseismic period, and trigger earthquake nucleation, we modelled fluid flow through a mature fault zone within the TE. Our model's parameters are the structure and permeability (k) of the major fault zone, as inferred from the integration of field observations and k laboratory data. The fault zone architecture is given by a distinct fault core, up to 3 meters thick, of very fine-grained fault gouge and cataclasites, surrounded by a geometrically complex damage zone (up to few tens of meters wide). The damage zone is characterized by adjacent zones of heavily- (dolostones) and non-fractured rocks (foliated anhydrites). The permeability of the fault core is inferred to be relatively low (k < 10E-18 m2), due to the presence of fine grained fault rocks. Where the connectivity of the fractured dolostone layers is high, the permeability of the damage zone is high (> 10E-17 m2) and controlled by the development of mesoscale fracture patterns. The permeability of the damage zone can be assumed to be as low as the values measured during our lab experiments on anhydrite rocks (k = 10E-17 - 10E-20 m2), where we observed the foliated Ca-sulphate rocks, with no mesoscale fracture patterns, completely surround the fractured blocks of dolostones. Our model results show that, during the seismic cycle, within portion of the fault zone where the fractured dolostone layers have a low connectivity, the lateral fluid flux across the fault zone is always lower than the vertical parallel fluid flux. We propose that overpressured patches cause dramatic fault weakening and trigger earthquake nucleation.