Shear fault formation in granite under high and transient co-seismic strain rates unveiled by ultra-fast X-ray imaging

During an earthquake a dynamic rupture propagates, causing fractures and permanent damage that modify the mechanical properties of the fault and its surroundings. Reciprocally, the energy dissipated by the mechanical transformation of the fault rock has a direct feedback on the dynamics of the seismic rupture. We reproduce and image these co-seismic damage processes with sub-microsecond time resolution through dynamic loading experiments on confined Westerly granite samples. Using synchrotron X-ray imaging and ultra-high-speed cameras, we observed that dynamic compression with confinement produces shear faults and a localized granulation of the rock, known as gouge, before significant slip occurs. This fragmentation process begins with volumetric damage, followed by shear deformation less than five microseconds later. Our results indicate that gouge formation is initiated by the dynamic rupture itself, which reduces the strength of a fault prior to other weakening mechanisms that may occur during the earthquake slip.