New weak fault model that does not require high pore pressure

• The proposed new weak fault model does not require high pore pressure . • In this model, isolated viscoelastic regions are distributed along the fault. • The viscoelastic regions continue to sustain normal stress even after relaxation. • This is similar to high pore pressures, and then the fault can be weakened. • The viscoelastic regions can include seismic faults within them. We propose a new weak fault model in which isolated viscoelastic regions are distributed along the fault. Numerical simulations using the finite element method show that the viscoelastic regions relax and the shear stress supported by them is applied to surrounding elastic regions after a time sufficiently longer than their relaxation time, while the normal stress continues to be supported by the viscoelastic regions, and then the normal stress in the elastic regions remain unchanged. Since the shear stress is amplified but the normal stress remains unchanged in the elastic regions, a macroscopic weakening of the fault occurs even under a constant coefficient of friction. The fault can be weakened without assuming high pore pressure. As a result of examining the effect of the geometry of the viscoelastic regions on the fault strength by changing their shape and spatial distribution in various ways, we found that the fault strength decreases as the ratio of the area of the elastic regions remaining unrelaxed to the total area of the fault decreases. It is known that faults can be weakened by fault rocks such as clay minerals, but the frictional properties of these fault rocks are basically velocity strengthening, making it difficult to weaken seismic faults. The fault model in this study is a model for deformation characteristics of the host rock around a fault, which does not place any constraints on the frictional properties of the fault, and thus can weaken a seismic fault.