Off‐Fault Damage Characterization During and After Experimental Quasi‐Static and Dynamic Rupture in Crustal Rock From Laboratory P Wave Tomography and Microstructures

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.