Cracked media, Poisson's ratio and the structure of the upper oceanic crust

Theoretical models of water-filled cracks within a solid are in general agreement, and indicate that the Poisson's ratio of the composite material will typically increase for thin cracks with aspect ratios less than 1/200, but will decrease for thick cracks with aspect ratios between 1/20 and ½. Near-spherical pores cause little change in Poisson's ratio. This is true both for randomly oriented, isotropic crack models, and, at certain directions, for aligned crack models which predict seismic anisotropy. Observed anomalous values of Poisson's ratio in oceanic Layer 2 can often be explained using these models as an effect of cracks, and do not necessarily require the presence of rocks of unusual composition. In particular, the low Poisson's ratios reported by Spudich & Orcutt (1980) and Au & Clowes (1984) can approximately, but not always exactly, be fitted with a model consisting of thick cracks within higher Poisson's ratio rock more typical of the upper oceanic crust. However, this interpretation is near the limit of existing theories of cracked material, so other explanations for the anomalous observations must also be considered.