Structural, frictional and fluid flow heterogeneities of crustal faults and implications for slip behaviour

Cristiano Collettini, Sapienza University of Rome

Wednesday, April 7, 2021 at 10:30 AM

Online-only seminar via Microsoft Teams
David Lockner

For years, there have been lines of evidence that inform the weak vs. strong fault debate. Strong faults, or fault segments, are mainly characterized by a granular load-bearing structure, LBS, with a frictional strength controlled by the Byerlee’s rule of friction. With increasing strain, localization along a principal slip zone favors the passage from rate strengthening to rate weakening behavior. Along other faults, or fault segments, fluid-assisted reaction softening promotes the development of interconnected phyllosilicate networks, IFN, that produce a significant reduction in friction and a marked rate-strengthening behaviour. LBS are prone to develop relatively high permeabilities (k > 10-16 m2), in particular at low effective normal stresses and within the damage zone, whereas IFN tend to reduce permeability significantly (k < 10-19 m2). Fluid pressure stimulations conducted on laboratory faults show that: 1) within the LBS fluid pressure steps easily diffuse within the fault and promote acceleration of slip that spontaneously evolves into dynamic failure; 2) in the IFN fluid pressure steps are associated with periodic accelerations and decelerations, that seem to be modulated by fluid pressure diffusion within the shear zones. In addition, for high levels of pressurization, slip velocity remains slow (i.e. v~200 µm/s), never approaching dynamic slip rates. Our integrated dataset of structural and mechanical data from natural and experimental structures indicate that fault slip behaviour and instability are strongly controlled by the coupling between structural, frictional and permeability properties of faults.

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