Off-fault damage characterisation during and after experimental quasi-static and dynamic rupture in crustal rock from laboratory P-wave tomography and microstructures

Abstract

Elastic strain energy released during shear failure in rock is partially spent as fracture energy $\Gamma$ to propagate the rupture further. $\Gamma$ is dissipated within the rupture tip process zone, and includes energy dissipated as off-fault damage, $\Gamma_\mathrm{off}$. 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 $\Gamma_\mathrm{off}$ 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 Lanh\'elin granite under triaxial conditions. We quantify the change in elastic moduli around the fault from time-resolved 3D $P$-wave velocity tomography obtained during and after failure. We measure the off-fault microfracture damage after failure. From the tomography, we observe a localised maximum 25\% drop in $P$-wave velocity around the shear failure interface for both quasi-static and dynamic failure. Microfracture density data reveals 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. $\Gamma_\mathrm{off}$ obtained from off-fault microfracture measurements is around 3 kJm$^{2}$ for quasi-static rupture, and 5.5 kJm$^{2}$ for dynamic rupture. We argue that rupture velocity determines damage zone width for slip up to a few mm, and that shear fracture energy $\Gamma$ increases with increasing rupture velocity.