Seismic velocities in rock subjected to axial loading up to shear fracture

The P and S velocities along three mutually perpendicular directions are measured as a function of the axial load on specimens of Indiana limestone. The specimens are rectangular prisms, and variable axial load is applied in the vertical direction parallel to the long dimension (Z axis). A clamp is used to hold the specimen firmly along a transverse direction (Y axis). Piezoelectric transducers of the compressional and shear type are used for both transmitting and receiving the seismic signal and are mounted on opposite faces of the specimen so that seismic velocities can be measured along the three orthogonal directions (Z, Y, and X axes). Seismograms showing the arrivals of P and S waves are obtained while the axial load is increased in small increments until shear fracture takes place. An examination of seismograms shows that prior to rupture both P and S velocities decrease in all three directions but by quite different amounts. The ratio of P to S velocities, ξ, remains nearly constant along the Z axis, decreases somewhat along the Y axis, and drops considerably along the X axis. These results, interpreted as being due to prefailure dilatancy, provide qualitative information on the anisotropic behavior of ξ, an important premonitory indicator of earthquakes.