STATE OF CRUSTAL STRESS INFERRED FROM IN SITU STRESS MEASUREMENTS

Abstract

The purpose of this paper is to show, from the results of in situ stress measurements using the stress relief method, to what extent we can infer the state of tectonic stress in the upper crust. The three-dimensional stress state at a measuring site is expressed by the stereographic projection of maximum shear plane using the same technique as for fault plane solution, and by diameters of double circles indicating maximum shear and mean stress magnitudes. These illustrations can be used to discriminate sites which do not reflect real tectonic stress. Horizontal stress orientations coincide fairly well with the tectonic stress field estimated from fault plane solutions, crustal deformation, and orientation of active faults. Horizontal stress magnitudes at each site are therefore corrected to a depth of 300 m using the standard stress gradient with depth and then compared with the results at the hydrofracturing sites in the Kanto-Tokai area for examination of local stress magnitude. There is a tendency that shear stress values obtained by the hydrofracturing method are low compared with those by the stress relief method. The relation of shear stress to mean stress is also interpreted graphically to determine the present stress state, and compared with a critical fracture condition in the upper crust. Whether the measured stress values indicate the real crustal stress state or not, may be inferred from these methods which also enable us to compare the regional differences of stress state.