Path and Slip Dependent Behavior of Shallow Subduction Shear Zones During Fluid Overpressure.

Elevated pore fluid pressure is proposed to contribute to slow earthquakes along shallow subduction plate boundaries. However, the processes that create high fluid pressure, disequilibrium compaction and dehydration reactions, lead to different effective stress paths in fault rocks. These paths are predicted by granular mechanics frameworks to lead to different strengths and deformation modes, yet granular mechanics do not predict their effects on fault stability. To evaluate the role of fluid overpressure on shallow megathrust strength and slip behavior, we conducted triaxial shear experiments on chlorite and celadonite rich gouge layers. Experiments were conducted at constant temperature (130 and 100°C), confining pressure (130 and 140 MPa), and pore fluid pressures (between 10 and 120 MPa). Fluid overpressure due to disequilibrium compaction was simulated by increasing confining and pore fluid pressure synchronously without exceeding the target effective pressure, whereas overpressure due to dehydration reactions was simulated by first loading the sample to a target isotropic effective pressure and then increasing pore fluid pressure to reduce the effective pressure. We find that the effects of fluid pressure and stress path on the mechanical behavior of the chlorite and celadonite gouges can generally be described using the critical state soil mechanics (CSSM) framework. However, path effects are more pronounced and persist to greater displacements in chlorite because its microstructure is more influenced by stress path. Due to its effects on microstructure, the stress path also imparts greater control on the rate‐dependence of chlorite strength, which is not predicted by CSSM. Plain Language Summary: High pore fluid pressure has been linked to complex slip modes along subduction plate boundary faults, such as low frequency earthquakes and slow slip. However, the role that pressurized fluids play in controlling these slip modes remains unclear. Along shallow subduction faults (<10 km), the source of pore fluid overpressure transitions with increasing depth from disequilibrium compaction of sediments to fluid release due to dehydration reactions like the smectite to illite transition. Using deformation experiments, we studied how different stress paths simulating these two processes affect the mechanical, frictional, and microstructural properties of shallow subduction fault rock at high fluid pressure. Key Points: Effective stress path and rock composition control effects of high pore fluid pressure on fault constitutive behaviorStress path effects on frictional behavior are more pronounced in simulated chlorite gouge than celadonite gouge due to differences in microstructureChlorite‐rich faults stabilize with increasing displacement at high pore fluid pressure and slow slip speeds [ABSTRACT FROM AUTHOR]