Frictional properties of the décollement in the shallow Nankai Trough: Constraints from friction experiments simulating in-situ conditions and implications for the seismogenic zone

Along subduction zone megathrust faults, the transition from smectite to illite with depth occurs at temperatures of ∼150°C and has long been hypothesized to be one of the dominant controls on the updip limit of the seismogenic zone. Despite this, there have been few experimental studies performed at in-situ conditions to test the frictional stability behavior of subduction zone faults across the smectite-illite transition. In this study, we simulated in-situ temperature-pressure-mineral conditions along the shallow plate boundary in the Nankai Trough, and conducted friction experiments to further constrain the role of smectite-illite transition in seismogenesis in subduction zones. We found that the friction coefficient of the simulated sediment increased with the progress of illitization and followed the Reuss average of friction coefficients of the mineral phases. The obtained friction coefficients and the Coulomb wedge model inferred that the pore pressure conditions around the plate boundary may be over-pressured but not as high as lithostatic pressure. Higher friction coefficients of sediment due to diagenetic processes would be required to sustain pore pressure close to lithostatic along the plate boundary. The velocity dependence of friction coefficient (a−b) was always positive (i.e., velocity strengthening, not seismogenic) except for slip rates less than ∼10 μm/s at 171°C where the smectite-illite transition has almost completed. Therefore, shallow slow earthquakes that occur beneath the outer prism in the Nankai Trough cannot be explained only by the frictional instability of unconsolidated sediment. Since the smectite-illite transition mostly completes at ∼30 km landward from the trough axis, the transition from velocity strengthening to velocity weakening observed at 171°C is caused by temperature-dependent frictional behavior of illite-rich material. Hence, the updip limit of the seismogenic zone in subduction zones is mainly controlled by the frictional properties of illite-rich material, not by the smectite-illite transition.