Energy Partitioning, Dynamic Fragmentation, and Off‐Fault Damage in the Earthquake Source Volume

Abstract

AbstractSeismological fracture or breakdown energy represents energy expended in a volume surrounding the advancing rupture front and the slipping fault surface. Estimates are commonly obtained by inverting ground motions and using the results to model slip on the fault surface. However, this practice cannot identify contributions from different energy‐consumption processes, so our understanding of the importance of these processes comes largely from field‐ and laboratory‐based studies. Here, we use garnet fragment size data to estimate surface‐area energy density with distance from the fault core in the damage zone of a deeply exhumed strike‐slip fault/shear zone. Estimated energy densities per fragmentation event range from 2.87 × 103 to 2.72 × 105 J/m3 in the outer and inner portions of the dynamic damage zone, respectively, with the dynamic zone being inferred from the fractal dimensions of fragment size distributions and other indicators. Integrating over the ∼105 m width of the dynamic damage zone gives fracture surface‐area energy per unit fault area ranging from a lower bound of 6.63 × 105 J/m2 to an upper bound of 1.63 × 107 J/m2 per event. This range overlaps with most geological, theoretical, and kinematic slip‐model estimates of energy expenditure in the source volume for earthquakes characterized by seismic moments >1017 N·m. We employ physics‐based fragmentation models to estimate equivalent tensile strain rates associated with garnet fragmentation, which range from 5.42 × 102 to 1.04 × 104 s−1 per earthquake in the outer and inner portions of the dynamic damage zone, respectively. Our results suggest that surface‐energy generation is a nonnegligible component of the earthquake energy budget.