Estimation of regional stress by FEM for a heterogeneous rock mass with a large fault

Defining regional stress for a heterogeneous rock mass composed of different rock bodies and a large fault as a displacement boundary condition that is consistent with the mean trend of the in situ stress, we propose a method for determining regional stress from limited stress data by using a 3D finite element method (FEM). First, this method was applied to a small-scale heterogeneous model with a large planar fault that penetrates the region, to clarify the estimation accuracy and the effects of poor modeling and incomplete stress data. The results showed that (1) the method proposed in this study can accurately estimate the stress distribution in the region under consideration except at the bottom when the sliding area of the fault is small, (2) the reproducibility of the given stress can be used to estimate appropriate values of the normal and shear stiffnesses of the fault when the fault is homogeneous, and (3) both incomplete stress data and the assumption that the fault does not slide decrease the accuracy of stress estimation while the given stresses are approximately reproduced. Next, the method was applied to the Tono area, Japan, where a large fault with a complex geometry, called the Tsukiyoshi fault, exists. The results obtained under the assumption that the Tsukiyoshi fault does not slide showed that (1) the stress distribution was estimated reasonably well only for a narrow region that contained the boreholes at which the stress used in the inverse analysis was measured, (2) the stress in the area shows a large scatter and a non-linear trend with depth, which is mainly caused by the Tsukiyoshi fault, and (3) when a fault is heterogeneous, comprehensive examination for the states of the stress and displacements on the fault is needed to obtain a reasonable estimation for the regional stress.