Fluid inclusion evidence of co-seismic fluid flow induced by dynamic rupture. In: Fault Zone Dynamic Processes: Evolution of fault properties during seismic rupture

Abstract

Seismogenic fault fracturing can create considerable fracture permeability in and around fault zones initiatinglarge fluxes of fluid, particularly at fault terminations and dilatational jogs. In this work we show that fluids canalso be channeled and potentially mixed through a network of interconnected high‐angle microfractures generatedby transient stress perturbations associated with a passing earthquake rupture. By using the orientation,chemical composition, and salinity in ca. 200 fluid inclusions trapped in healed microfractures across the damagezone of a crustal‐scale fault, we show that high‐angle healed microfractures close to the damage zone/faultcore boundary host high CO2 contents and a wide range of salinities. The width of this zone is ~35m. The high‐angle microfractures are interpreted as having formed from the passage of earthquake ruptures as they areconsistent with the inferred stress field from dynamic rupture models. We infer that the rapid creation of thefracture network leads to phase separation and fluid mixing, resulting in the highly variable fluid chemistry.The results suggest pore‐fluid flow fluctuations are not only restricted to geometrical irregularities along faults,but also to regions of the damage zone close to a passing earthquake.