Changes in seismic velocity and attenuation during deformation of granite

Abstract

The seismic velocity and attenuation were measured for P, S⊥, and S∥ waves traveling through a sample of Westerley Granite as it was deformed to failure under a confining pressure of 500 bars. All waves traveled in a direction normal to the axis of maximum compression, with S∥ polarized parallel to the axis and S⊥ polarized normal to it. By the time failure occurred, all seismic velocities had decreased by 12–30%. Amplitudes of the S⊥ and P waves decreased by approximately 30%. A remarkable result of the experiment is that the amplitude of the S∥, wave increased throughout the experiment until near failure. The amplitude at 90% failure strength was more than twice the value under hydrostatic pressure alone. These results are explained by analyzing how elastic moduli and energy dissipation due to frictional sliding at cracks are affected by anisotropic crack distribution developed in the sample as axial stress is applied.