Off-Fault Damage Characterization 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 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.