MODELING THE COMPLIANCE OF CRUSTAL ROCK .1. RESPONSE OF SHEAR-WAVE SPLITTING TO DIFFERENTIAL STRESS

We show that seismic shear waves may be used to monitor the in situ st ress state of deep inaccessible rocks in the crust. The most widesprea d manifestation of the stress-related behaviour of seismic waves is th e shear-wave splitting (shear-wave birefringence) observed in almost a ll rocks, where the polarizations of the leading split shear waves are usually subparallel to the direction of the local maximum horizontal stress. It has been recognized that such shear-wave splitting is typic ally the result of propagation through distributions of stress-aligned fluid-filled microcracks and pores, known as extensive-dilatancy anis otropy or EDA. This paper provides a quantitative basis for the EDA hy pothesis. We model the evolution of anisotropic distributions of micro cracks in triaxial differential stress, where the driving mechanism is fluid migration along pressure gradients between neighbouring microcr acks and pores at different orientations to the stress held. This lead s to a non-linear anisotropic poroelasticity (APE) model for the stres s-sensitive behaviour of fluid-saturated microcracked rocks. A compani on paper shows that APE modelling matches a range of observed phenomen a and is a good approximation to the equation of state of a stressed f luid-saturated rock mass.