5:15 PM - 7:15 PM
[SCG59-P01] Experimental constrain on water penetration velocity along outer-rise faults
Keywords:Outer-rise fault, Peridotite, Permeability, Triaxial deformation test, Hydrostatic test
Outer-rise earthquakes occur on the seaward of the trench, which is subjected to tensile forces and develops a normal fault. The outer-rise fault reaches the mantle at a depth of about 40 km below the seafloor, and it is thought that seawater penetrates along the fractures and that the peridotite around the fault is serpentinised. In this study, in order to clarify the seawater penetration velocity in the outer-rise fault, triaxial deformation experiments were performed, and the permeability was measured during the experiments and after fracturing. Hydrostatic experiments were also performed after the deformation experiments to investigate the pressure effect on permeability. Based on these results, the velocity of seawater penetration along the outer-rise fault is discussed.
In this study, Horoman peridotite collected from the Hidaka Mountains, Hokkaido, Japan, was used as the sample. The main constituent minerals are olivine, orthopyroxene and Cr-spinel, and they are almost unaltered. The experiments were performed using an intra-vessel deformation and fluid flow apparatus. Experiments were performed at room temperature under a constant strain rate, confining pressure of 5-10 MPa and pore pressure of 1.0-1.7 MPa. Axial and radial strains were measured by two strain gauges and the permeability was calculated based on the constant differential pressure method using nitrogen gas. Experiments were also performed under hydrostatic pressure after the sample fractured, and while the confining pressure varied from 5-200 MPa, the permeability was measured using water as the pore fluid to investigate its pressure effect.
The experimental results show that the peridotite exhibits elastic behavior from the initial to the middle stages of deformation, and that the samples show dilatancy under high differential stress just prior to failure. The maximum differential stresses under a confining pressure of 5 MPa were 312-360 MPa and under a confining pressure of 10 MPa was 452 MPa. The dilatancy initiation stress was 61% of the maximum differential stress under a confining pressure of 5 MPa and 76% of the maximum differential stress under a confining pressure of 10 MPa. The initial permeability was 10-20-10-19 m2 and the permeability during deformation was almost constant, with a sharp increase just before failure. The post-failure permeability was 10-16-10-15 m2, showing a change in permeability of 3-5 orders of magnitude before and after failure. In the experiments under hydrostatic pressure performed after sample fractured, the permeability tended to decrease as the confining pressure increased, with a permeability of 10-16-10-17 m2 at a confining pressure of around 100 MPa. It should be noted that the pressure effect on permeability was not constant, but decreased significantly until the confining pressure was ~40 MPa, and then showed a slowly decreasing trend. It has been reported that the permeability of fractured rock decreases exponentially with increasing effective normal stress and changes to a linear relationship at a certain stress. In the present study, it is thought that the relationship changes to a linear relationship at around 40 MPa of confining pressure, and the flow path is disconnected, and a disconnected flow is generated.
Based on the permeability of these fractured peridotites, the velocity of seawater penetration along the outer-rise fault was calculated using Darcy's law. In the calculations, the water pressure at a depth of 4,000 m is assumed to be the driving force of fluid flow, while the water pressure at the tip of the fault is assumed to be zero. Based on the percolation model, the depth at which seawater penetrates along the outer-rise fault is calculated. The seawater penetration reaches 60-90 km in one million years, indicating that seawater can penetrate deep into the outer-rise fault. It is inferred that the peridotite around the outer-rise fault reacts with seawater and forms serpentinite.
In this study, Horoman peridotite collected from the Hidaka Mountains, Hokkaido, Japan, was used as the sample. The main constituent minerals are olivine, orthopyroxene and Cr-spinel, and they are almost unaltered. The experiments were performed using an intra-vessel deformation and fluid flow apparatus. Experiments were performed at room temperature under a constant strain rate, confining pressure of 5-10 MPa and pore pressure of 1.0-1.7 MPa. Axial and radial strains were measured by two strain gauges and the permeability was calculated based on the constant differential pressure method using nitrogen gas. Experiments were also performed under hydrostatic pressure after the sample fractured, and while the confining pressure varied from 5-200 MPa, the permeability was measured using water as the pore fluid to investigate its pressure effect.
The experimental results show that the peridotite exhibits elastic behavior from the initial to the middle stages of deformation, and that the samples show dilatancy under high differential stress just prior to failure. The maximum differential stresses under a confining pressure of 5 MPa were 312-360 MPa and under a confining pressure of 10 MPa was 452 MPa. The dilatancy initiation stress was 61% of the maximum differential stress under a confining pressure of 5 MPa and 76% of the maximum differential stress under a confining pressure of 10 MPa. The initial permeability was 10-20-10-19 m2 and the permeability during deformation was almost constant, with a sharp increase just before failure. The post-failure permeability was 10-16-10-15 m2, showing a change in permeability of 3-5 orders of magnitude before and after failure. In the experiments under hydrostatic pressure performed after sample fractured, the permeability tended to decrease as the confining pressure increased, with a permeability of 10-16-10-17 m2 at a confining pressure of around 100 MPa. It should be noted that the pressure effect on permeability was not constant, but decreased significantly until the confining pressure was ~40 MPa, and then showed a slowly decreasing trend. It has been reported that the permeability of fractured rock decreases exponentially with increasing effective normal stress and changes to a linear relationship at a certain stress. In the present study, it is thought that the relationship changes to a linear relationship at around 40 MPa of confining pressure, and the flow path is disconnected, and a disconnected flow is generated.
Based on the permeability of these fractured peridotites, the velocity of seawater penetration along the outer-rise fault was calculated using Darcy's law. In the calculations, the water pressure at a depth of 4,000 m is assumed to be the driving force of fluid flow, while the water pressure at the tip of the fault is assumed to be zero. Based on the percolation model, the depth at which seawater penetrates along the outer-rise fault is calculated. The seawater penetration reaches 60-90 km in one million years, indicating that seawater can penetrate deep into the outer-rise fault. It is inferred that the peridotite around the outer-rise fault reacts with seawater and forms serpentinite.