Seismic Excitation of Rough, Confined Fractures

We have carried out numerical experiments to investigate the seismic response of rough fractures located in ambient stress fields similar to those in the crust. We use a particle-based approach in which the geological medium is considered to be made up of a large number of frictionless circular particles, which represent atoms, grains of sand, or blocks of crustal rock, depending on the scale. These particles are connected by linearly elastic bonds whose stiffnesses control the elastic properties of the medium. Fractures are represented in this model as zones of greater compliance than the host material. This is achieved by assigning lower, or zero, stiffnesses to bonds crossing the fracture. In contrast with most current fracture models, the fracture stiffnesses in tension and compression can be varied independently, allowing fractures with various levels of cohesion to be modelled. This scheme has been shown to be in agreement with analytical models for wave propagation in piecewise heterogeneous materials (Toomey & Bean, Geophys. J. Int., 141, 595, 2000). The scattered wavefield and interface waves produced during seismic excitation of smooth fractures have been shown to match those predicted by the displacement discontinuity theory for fractures (Toomey, Bean & Scotti, submitted to Geophys. Res. Lett., 2001). In addition, rock compression and shear can easily be modelled, allowing wave propagation through confined materials to be investigated. A large number of studies of fracture surface roughness have shown that fractures and joints exhibit self-affine scale invariance. We have generated synthetic fractures with the statistical properties of real fractures and examined their response to stress in the particle-based numerical scheme. non-linear stress-strain behaviour similar to that observed in laboratory experiments on rough fractures occurs. After stressing the fractures up to crustal stress levels, seismic waves are propagated through the model and the interaction of the fractures with the dynamic stress field is examined. The influence of the ambient stress field and fracture orientation with respect to the maximum principal stress on the diffracted wavefield and fracture interface wave generation is examined. Conditions under which fault slip may occur in response to dynamic stresses are investigated.