Numerical fracture simulation using Hamiltonian Particle Method -Relationship of parallel faulting to stress filed and elastic parameters of rock mass-

Echelon faults are a group of parallel shear failures of rock that have a certain angle to stress axis. These parallel cracks can be observed as various sizes from plate-scale to laboratory-scale. In the area surrounding Japan, one of the typical example is found in the Izu Peninsula and the offshore area. In these areas, the conjugate parallel faults can be also observed. The mechanism and the formation of these parallel and conjugate faults are not well investigated, and there still remains an important geophysical subject. In the Izu Peninsula, the crust mainly suffers the compressional stress force by the subduction of the Philippine Sea Plate and strike slip forces of two large shear faults. In this study, we conduct numerical simulations of rock mass failure under various conditions to discuss the forming mechanism of these parallel and conjugate fractures. If we find the relationship between the formation of echelon faults and stress field applied to rock mass, we could infer stress field loaded to the crust from the pattern of faults. For numerical simulation, the Finite Difference Method (FDM) and the Finite Element Method (FEM) are widely used to solve solid deformation problems. In these methods, the failure at faults or cracks, however, would not be well simulated when the displacement becomes large or the grid-based structure is broken. On the other hand, particle methods are free from these difficulties. Therefore, we use the Hamiltonian Particle Method (HPM), i.e., one of the particle methods to simulate the formation of echelon faults to investigate the nucleation conditions. We assume a rectangular plate and change the type of forces acting on the plate for simplifying Izu area. The model represents a two-dimensional plane strain. In Izu area, the oceanic crust is composed mainly of basalt. Thus, we assume that the parameters of the rock mass are basaltic. The density and Young's modulus of the model are determined to match those of basalt. In general, rock is a heterogeneous material, and this heterogeneity causes failure of rock Therefore, we consider the Weibull distribution to the strength of the particles as the basis for heterogeneity. Among a lot of failure criteria, the Mohr-Coulomb criterion is commonly used because of its simplicity, defined only by two principal stresses. Therefore, we adopt this criterion for the shear failure. We simulate the rock failure under four loaded conditions. As a result, in the case of axial compressive force or single shear force is only acting on the rock mass, the main shear faults dominantly propagates and parallel or conjugate fracture does not appear. Adding the confining pressure not to expand perpendicular direction to axial direction encourages propagation of the parallel or conjugate fractures. On the other hand, if the shear force acts on the rock mass, the parallel fractures propagation corresponding to shear direction is triggered, but the conjugate fractures propagation is prevented.