FLUID OVERPRESSURE AND EARTHQUAKE TRIGGERING on DOLOMITES AND ANHYDRITES BEARING FAULTS: INTEGRATION OF FIELD AND LABORATORY STUDIES

The integration of seismic reflection profiles with well-located earthquakes shows that the mainshocks of the 1997–1998 Umbria–Marche seismic sequence (Central Italy) nucleated at a depth of ∼6 km on normal faults within the Triassic Evaporites. In the same lithology, made of interbeds of dolomites and anhydrites, two deep boreholes (∼4–5 km) encountered CO2 at near lithostatic pressure. In order to investigate the deformation processes operating at depth in the source region, we have combined field studies on a major evaporite-bearing normal fault (the Roccastrada fault) with mechanical and permeability data from rock deformation experiments on anhydrite rocks. The Roccastrada fault zone structure is characterized by a 5–6 m thick fault core, which appears to be zoned. The inner fault core is made of fine-grained fault rocks (∼1 m thick), with deformation localized along continuous and straight slip surfaces associated with a dolomite-rich cataclasite (brittle deformation). The outer fault core is mainly characterized by distributed deformation accommodated by a fault parallel fabric consisting of interbeds of cataclastic dolostones and foliated Ca-sulphate rocks. Mechanical data obtained from triaxial loading tests show that the transition from localized to distributed deformation occurs at effective pressures of about 20 MPa. The permeability measured under hydrostatic stress conditions, before loading, is generally low and ranges between 10⁻²¹≤ k≤ 10⁻¹⁹ m². During sample loading, the permeability increases up to 3 (prior to brittle localized failure) and 2 (prior to distributed ductile failure) orders of magnitude, with measured k values of 10⁻¹⁷ m² and 10⁻¹⁸ m², respectively.

The combination of field observations and permeability measurements suggests that progressive deformation within the fault core leads to the development of fault parallel dolomite-rich cataclastic layers. Their reactivation coupled with transient fluid overpressure, that is favored by the permeability values of the fault zone structure, can produce embrittlement and localization of brittle deformation. Therefore in the active area of the Umbria–Marche Apennines deep-seated CO2-rich fluids can be trapped at seismogenic depths within evaporite-bearing faults and can potentially promote earthquake nucleation.