Evolution of co-seismic off-fault damage towards pulverisation

Pulverisation of rocks occurs through dynamic processes associated with earthquake ruptures. At the well-known pulverised rock outcrops along the San Andreas Fault (SAF), the intensity of pulverisation was shown to be highly heterogeneous on the outcrop (m-10s of m) scale, although detailed documentation of this heterogeneity is still lacking. Here we derive a standardized classification method for pulverisation intensity applicable to field observations of rocks that have been pulverised or dynamically shattered. We apply this classification to outcrops along the mature SAF and along a smaller displacement fault, the Borrego Fault (BF). Centimetre-scale mapping revealed damage heterogeneity at both locations, but while pulverisation intensity is heterogeneous at the SAF, it is mostly the meso-fracture damage that causes heterogeneity at the BF, although sporadic pulverisation is observed adjacent to the fault core. We postulate that the pattern of heterogeneity in pulverisation intensity along the SAF evolved from an originally patchy distribution of meso-fractures as observed on the BF. We develop a damage-evolution model, in which heterogeneous meso-fracture damage originating from damage focussed at subsidiary faults yields a heterogeneous distribution of elastic properties and therefore a heterogeneous strain-rate field of a future passing rupture. As pulverisation depends on strain rate, the heterogeneous strain rate field is then responsible for the heterogeneous pulverisation intensity on the matured fault. This damage-evolution model integrates field observations on damage intensity and heterogeneity, results from laboratory experiments on pulverisation strain-rate thresholds, and theoretical considerations on the dynamic origin of pulverised fault zone rocks.(C) 2022 Elsevier B.V. All rights reserved.