Seismic evidence for fluid-driven deformation

The behaviour of seismic shear waves shows that fluid–rock interactions control both low-level deformation of the intact rock mass before fracturing takes place, and the fracturing or faulting process itself in deep in situ rock. The splitting (birefringence) of shear-waves directly indicates that the pre-fracturing deformation of intact unfractured rock is the result of fluid movement along pressure gradients between adjacent microcracks at different orientations to the stress field. This is the mechanism for low-level pre-fracturing deformation of almost all in situ rocks. Further seismic evidence shows that fracturing and faulting at depth only occur when pore-fluid pressures on seismically active fault planes are sufficiently high to relieve frictional forces and allow asperities to be overcome. This is comparatively direct evidence that fluids control low-level small-scale (pre-fracturing) deformation of the intact rockmass, and that most if not all fracturing only occurs when fluid pressures on the fault plane are critically high.