Abstract
AbstractElastic strain energy released during shear failure in rock is partially spent as fracture energy Γ to propagate the rupture further. Γ is dissipated within the rupture tip process zone, and includes energy dissipated as off‐fault damage, Γoff. Quantifying off‐fault damage formed during rupture is crucial to understand its effect on rupture dynamics and slip‐weakening processes behind the rupture tip, and its contribution to seismic radiation. Here, we quantify Γoff and associated change in off‐fault mechanical properties during and after quasi‐static and dynamic rupture. We do so by performing dynamic and quasi‐static shear failure experiments on intact Lanhélin granite under triaxial conditions. We quantify the change in elastic moduli around the fault from time‐resolved 3‐D P wave velocity tomography obtained during and after failure. We measure the off‐fault microfracture damage after failure. From the tomography, we observe a localized maximum 25% drop in P wave velocity around the shear failure interface for both quasi‐static and dynamic failure. Microfracture density data reveal a damage zone width of around 10 mm after quasi‐static failure, and 20 mm after dynamic failure. Microfracture densities obtained from P wave velocity tomography models using an effective medium approach are in good agreement with the measured off‐fault microfracture damage. Γoff obtained from off‐fault microfracture measurements is around 3 kJ m2 for quasi‐static rupture, and 5.5 kJ m2 for dynamic rupture. We argue that rupture velocity determines damage zone width for slip up to a few mm, and that shear fracture energy Γ increases with increasing rupture velocity.
Highlights
During shear failure in rock, stored elastic strain energy is partly released as radiated energy Er and mostly dissipated on and around the fault interface as latent heat and new fracture surface area through a plethora of dissipative processes
The slip on the fault δ, calculated from the axial displacement data corrected for machine stiffness and for the stiffness of the intact rock, was 0.83 mm at the end of the quasi‐static rupture experiment, 2.88 to 3.22 mm after dynamic failure, and 1.93 to 2.44 mm after dynamic failure in mixed rupture experiments (Table 1)
Before we provide an estimate for Γsouffrf, achieved by combining the damage zone width and microfracture density, we assess whether the difference in damage zone width obtained for quasi‐static and dynamic rupture is an effect of rupture velocity or an effect of the difference in accumulated fault slip
Summary
During shear failure in rock, stored elastic strain energy is partly released as radiated energy Er (i.e., seismic waves) and mostly dissipated on and around the fault interface as latent heat and new fracture surface area through a plethora of dissipative processes. Breakdown work Wb is a collective term of energies dissipated in addition to Ef, and primarily includes dissipative processes that reduce the strength of the fault interface toward the residual friction. This includes comminution, flash heating (Brantut & Viesca, 2017), and thermal pressurization (Viesca & Garagash, 2015), and includes energy dissipated toward propagating the rupture tip and energy dissipated by deformation outside the principal slip zone (off‐fault deformation). Γ is called the shear fracture energy and is the energy dissipated within a process zone surrounding the rupture tip to overcome cohesion of the material and propagate the rupture by a unit area (Freund, 1990). Measurements for material parameter Γ are of the order of 104 J m−2 for initially intact crystalline low‐porosity rock under upper crustal conditions
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