Natural Slip Surfaces in Fault Cores of Seismogenic Carbonate Faults

Observations from drilled fault cores of major active fault zones show that seismic slip is localized within narrow slip zones (1-5 mm) of fine-grained fault rocks. The evolution with slip rate and displacement of the frictional properties of the slip surfaces and the adjacent fault rocks, they develop within, controls the propagation of an earthquake. Recent friction experiments performed on a variety of rock types at seismic slip rates have shown a dramatic drop of frictional strength (f = 0.1 - 0.2) from the initial peak values in the Byerlee's range (f = 0.65 - 0.8). Despite an overwhelming number of laboratory data showing dynamic lubrication of experimental faults at seismic slip rates, the mechanical properties of natural slip zones and associated slip surfaces are still poorly understood. Furthermore, there is still a paucity of field observations from natural slip zones of active faults, which may be related to the thermally activated rupture propagation processes. We studied 11 fault segments of three well-exposed extensional fault systems in the northern Apennines of Italy: the Umbria Fault system, the L'Aquila Fault system and the Fucino Basin fault system. These fault systems are located along the NW-SE oriented seismic belt of the northern Apennines of Italy, which has produced moderate/large earthquakes (5<M<7) between 1915 and 2009. The studied faults propagate through the thick Mesozoic-Cenozoic carbonate sequence of the Apennines seismogenic layer. Field studies show that the large scale fault geometry is given by a damage zone and a fault core. The fault core/damage zone transition is sharp; most of the displacement is accommodated within the fault cores, which are from few to tens of meters thick. The fault cores internal architecture is complex and made of fault parallel domains (up to few meters thick) of fault breccias, fine to coarse grained cataclasites and fault gouges. Within the fault core, slip is further localized in narrow slip zones bounded on one side by principal slip surfaces, which appear as polished and striated planes. Slip localization has been observed both at the boundary between two different fault rock domains and within a homogeneous fault rock domain. Microstructural analyses will be performed on samples of fault rocks and slip zones, collected from the different fault core domains, to infer the dominant deformation mechanisms. Friction experiments will be performed, for a range of normal stresses (1-20 MPa), on the different type of fault rocks and the natural slip surfaces to characterize their frictional behaviour at constant (strain hardening/weakening) and variable (velocity weakening/strengthening) sub-seismic slip rates (microns/s) and at seismic slip rates (1 m/s). Grain size particle analyses and porosity data on fault rock samples will be used to quantitatively compare field and laboratory fault rocks.