Direction Of Minimum Principal Stress Research Articles

真三軸圧縮下でのウェスタリー花崗岩の弾性波速度異方性について

Seismic wave velocities as well as the three principal strains were measured in the rectangular prisms of Westerly granite (35×35×70mm) under general triaxial compressional state where the three principal stresses are different each others. Measurements were carried out under confining pressure of 50MPa, and under 100MPa and 150MPa intermediate stresses, both with 50MPa minimum principal stress. Compressional velocities for the three principal stress directions and two polarized shear velocities for the minimum principal stress direction were measured during the deformation of specimen.The P-wave velocity anisotropy between the intermediate and the minimum principal stress direction increased with increment of the intermediate principal stress. The velocity decrease of the two polarized S-waves in the minimum principal stress direction showed considerable difference under confining pressure experiment, while it became almost equal under the higher intermediate principal stress state. Using the relation between velocities and crack densities employed by SOGA et al. (1978), we calculated crack densities and average aspect ratios for the two types of flat spheroidal cracks whose plane are perpendicular to the intermediate and the minimum principal stress directions.The crack densities for both crack types was equal under confining pressure experiment. On the other hand, the crack density of the plane perpendicular to the intermediate principal stress was reduced remarkably under the intermediate stress conditions resulted from the preferred orientation of cracks with their plane perpendicular to the minimum principal stress. This perferred crack model also explains the equal velocity decrease of the two polarized S-waves under the three different principal stress state. The average aspect ratio of cracks with its plane normal to the intermediate principal stress becomes large under the general triaxial stress state, in comparison with that under confining pressure. This suggests that only thick cracks can survive when the intermediate principal stress acts normal to the crack plane. The calculated average aspect ratios are in line with those determined by HADLEY (1976).

Shear wave birefringence in dilating granite

Laboratory observations of ultrasonic shear waves indicate splitting along the direction of minimum principal stress of ∼11% for confined (20 MPa) and ∼16% for unconfined specimens of Westerly granite loaded in uniaxial stress. Dilatant strains increase birefringence except at > 80‐90% of the ultimate strength, when processes related to shear failure apparently become important. If seismic birefringence observed in the field can be related to anisotropic dilatant strains, these laboratory measurements of precursory velocity changes should prove useful in predicting earthquakes.