Abstract:
Geological causes of crustal anisotropy include regional fractures and cracks, isotropic heterogeneity or layering and material composition and textural properties. In addition, shear or metamorphic foliations in fault zones or structural terranes can serve as proxies for intracrustal deformation in a manner analogous to the lattice-preferred orientation of olivine produced by mantle shear. The primary factor in the production of crustal anisotropy is the relative angle between a seismic wave and the (dipping) symmetry axes of crustal material as either change along the propagation ray path. Complex and heterogeneous, crustal material is not necessarily oriented with vertical/horizontal symmetry axes, but is more normally tilted in accordance with structure. Calculation of the anisotropic response to material tilt for elastic seismic wave propagation requires rotation of the stiffness tensors describing the crustal material. We examine the alteration of anisotropic stiffness tensors and provide rotation directional cosines based on geological measurements. Material tilt produces strong effects in examples of 2-D and 3-D finite-difference synthetic seismograms that might otherwise be attributed to velocities or structure in other regions of the crust or mantle.
