11:30 AM - 11:45 AM
[SCG55-09] A weak fault model that does not require high pore pressure
Keywords:Fault strength, Viscoelastic, Pore pressure, Intraplate Eartquake
1. weak fault models
Since it was pointed out that the shear stress acting on the San Andreas fault is very small (Zoback, 1987), a number of observations have shown that the strength of not only faults at plate boundaries but also inland faults is small (e.g. Yoshida et al., 2014). The reason why the strength of faults estimated from observations is so small compared to the strength estimated from the friction coefficient of faults measured in rock friction experiments (Byerlee, 1978) is often explained by the fact that high pore pressure reduces the normal stress acting on the fault (Rice, 1992).Rice(1992) showed numerically that a continuous supply of fluid with high pore pressure from the lower crust of the San Andreas Fault results in smaller intensities. However, unlike plate boundaries, it is unlikely that a continuous supply of fluid with high pore pressure inland for a long period of time is possible. Fluid activity, presumably rising from depth, may be captured after a major earthquake, but their time scale is at best a few months (Sibson, 1988).
Analysis of fault rocks obtained from drilling of the San Andreas Fault has reported strength reduction due to slipstones generated in the serpentinite (Moore & Rymer, 2007). It has also been noted that the presence of water in clay minerals causes the clay to swell and reduce the normal stress acting on the fault (e.g. Wang & Mao, 1979). However, these fault materials generally exhibit velocity-enhancing frictional properties, making these ideas applicable to creeping faults, but difficult to apply to seismic faults.
Yamamoto et al. (2001, 2002) showed that in fault fracture zones, frictional forces can be reduced because vertical cracks exist in the fault, which support normal but not shear stresses. The point of this idea is that a fault is weakened if there are areas that support normal but not shear stresses. High pore pressure is just such a thing, but even viscoelastic materials are expected to relax after a long time and reach a similar state. In this report, the finite element method is used to simulate what happens to the strength of a fault after a long period of time when the gouge part of the fault is a viscoelastic material.
2. results of finite element calculations
A finite element model with a viscoelastic region on a patch near the fault was constructed using ABAQUS (2023). Specifically, regions with a rectangular cross-sectional shape in the fault orthogonal direction (called gouge regions) were periodically placed along the fault and their response after a long period of time was investigated. After initially setting the normal stress to a constant value, shear deformation was applied to the fault to determine at what point the fault began to slip. Several geometries were used, ranging from thin rectangular to square gouge regions, all of which were able to reproduce a reduction in macroscopic strength compared to the homogeneous case.
Since it was pointed out that the shear stress acting on the San Andreas fault is very small (Zoback, 1987), a number of observations have shown that the strength of not only faults at plate boundaries but also inland faults is small (e.g. Yoshida et al., 2014). The reason why the strength of faults estimated from observations is so small compared to the strength estimated from the friction coefficient of faults measured in rock friction experiments (Byerlee, 1978) is often explained by the fact that high pore pressure reduces the normal stress acting on the fault (Rice, 1992).Rice(1992) showed numerically that a continuous supply of fluid with high pore pressure from the lower crust of the San Andreas Fault results in smaller intensities. However, unlike plate boundaries, it is unlikely that a continuous supply of fluid with high pore pressure inland for a long period of time is possible. Fluid activity, presumably rising from depth, may be captured after a major earthquake, but their time scale is at best a few months (Sibson, 1988).
Analysis of fault rocks obtained from drilling of the San Andreas Fault has reported strength reduction due to slipstones generated in the serpentinite (Moore & Rymer, 2007). It has also been noted that the presence of water in clay minerals causes the clay to swell and reduce the normal stress acting on the fault (e.g. Wang & Mao, 1979). However, these fault materials generally exhibit velocity-enhancing frictional properties, making these ideas applicable to creeping faults, but difficult to apply to seismic faults.
Yamamoto et al. (2001, 2002) showed that in fault fracture zones, frictional forces can be reduced because vertical cracks exist in the fault, which support normal but not shear stresses. The point of this idea is that a fault is weakened if there are areas that support normal but not shear stresses. High pore pressure is just such a thing, but even viscoelastic materials are expected to relax after a long time and reach a similar state. In this report, the finite element method is used to simulate what happens to the strength of a fault after a long period of time when the gouge part of the fault is a viscoelastic material.
2. results of finite element calculations
A finite element model with a viscoelastic region on a patch near the fault was constructed using ABAQUS (2023). Specifically, regions with a rectangular cross-sectional shape in the fault orthogonal direction (called gouge regions) were periodically placed along the fault and their response after a long period of time was investigated. After initially setting the normal stress to a constant value, shear deformation was applied to the fault to determine at what point the fault began to slip. Several geometries were used, ranging from thin rectangular to square gouge regions, all of which were able to reproduce a reduction in macroscopic strength compared to the homogeneous case.