Insights on the geometry and mechanics of the Umbria–Marche earthquakes (Central Italy) from the integration of field and laboratory data

Abstract

The integration of seismic reflection profiles with well-located earthquakes shows that the mainshocks of the 1997–1998 Umbria–Marche seismic sequence 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), drilled northwest of the epicentral area, encountered CO 2 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 is a mature normal fault (inferred displacement > 100 m and exhumation from depths > 1 km) that dips in the range 40°–45°. The 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. The damage zone consists of foliated Ca-sulphate rocks (foliation almost perpendicular to the fault zone) and heavily fractured and boudinaged dolostones. Mechanical data obtained from triaxial loading tests on borehole-recovered anhydrites with different grain size and mesoscopic fabric, 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 m 2. 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 m 2 and 10 − 18 m 2, respectively. The integration of field observations with mechanical data allows us to propose a fault evolution model where fault initiation occurs as distributed deformation within the anhydrites, along fault zones dipping at ∼ 45°. With increasing displacement, dolostones are concentrated into the inner fault core. The ultimate result is a fault zone structure dipping at ∼ 45° , made of an inner fault core affected by brittle processes surrounded by a macroscopically ductile outer fault core. This fault zone evolution can explain the low rupture dip (38°–48°) of the Umbria–Marche 1997–1998 mainshocks. The combination of field observations and permeability measurements suggests a fault zone permeability low enough for fluid overpressures to develop. Therefore in the active area of the Umbria–Marche Apennines deep-seated CO 2-rich fluids can be trapped at seismogenic depths within evaporite-bearing faults and can potentially promote earthquake nucleation.