Direction dependence of fracture compliance induced by slickensides

Abstract

The effect of a fracture on seismic wave propagation can be represented in terms of the normal and shear fracture compliance. In many studies, fractures are assumed to have no preferential orientation for slip along the fracture plane. However, examination of fractures found in recovered cores or in exposed outcrops shows irregularities, such as hackles or slickensides, which are formed in the direction of joint/fault propagation. These irregularities, represented as sawtooth ridges, can be expected to facilitate movement in a particular direction more than the opposite direction, in a way that depends on the height and shape of the irregularities. As a result, fractured rocks should exhibit a different compliance for shear in one direction compared with shear in the opposite orientation. We used numerical modeling to offer a critical assessment of this phenomenon by directly calculating the change in normal and shear compliance in the presence of hackles. The effect of the geometry of the hackles, the friction coefficient between fracture surfaces, and the role of normal stresses on the ratio of normal-to-shear fracture compliance were evaluated.