Theoretical consideration on a relationship between tidal periods and permeability on the plate interface with high-pressure fluids

It is known that slow slip events (SSEs) occur in regions where the pore fluid pressure is higher at depths around 30 km. Because those fluids reduce the effective normal stress on the plate interface significantly, SSEs can be triggered by weak perturbing external stresses such as tidal stresses. Numerical simulation studies have suggested that temporal changes in the pore fluid pressure during an SSE affect slip behaviors. Moreover, some studies suggest that the high-pressure fluids may migrate upward along the plate interface as the slow slip progresses, which may weaken the fault strength there and trigger a large earthquake. From these reasons, it is important to reveal in which manner those high-pressure fluids behave during SSEs.

The responses of fault slips to periodic external stresses have been investigated by laboratory experiments and numerical simulations. However, most of them did not take the presence of pore fluids into account. In other words, how the responses of poroelastic media to periodic external stresses have not yet been fully understood. There is a claim that the occurrence of large earthquakes is correlated with long period external stresses such as the 18.6-year tide. However, amplitudes of longer period external stresses are generally much lower than those of shorter period ones. To answer whether such small stresses could trigger earthquakes, we should investigate whether the behaviors of the pore fluids and the slip can vary due to the difference in the periods of the applied external stresses.

In this study, we apply a periodic stress disturbance to a slow slip in a poroelastic medium composed of solid and liquid phases. Mathematically, we employ a two-dimensional model that assumes a quasi-static stress change and a rate- and state- friction law. We focus on the effective permeability within the fault fracture zone which governs the fluid behaviors and assess its frequency dependence on the applied period. We are currently formulating the basic equations prescribing the fluid flow and slow slip in such a case, mainly based on Suzuki and Yamashita (2007). In the presentation, we will show preliminary simulation results on the effective permeability.