Anisotropic Seismic Response of Crustal Tectonic Features Using 3D Finite Differences

Tectonic terranes within the crust often possess compositional changes and internal fabrics due to the deformational and metamorphic processes associated with tectonic modification. These material traits (composition, structural geometry, internal fabric) may alter seismic wave propagation as evidenced by anisotropic particle motions, travel time delays, and phase splitting. In addition, internal deformational fabrics may be oriented oblique to geological layer boundaries. All of these effects may produce anisotropic wave propagation. Observations of seismic anisotropy produced from within the crystalline crust are becoming increasingly common. Studies of crustal anisotropy are more complex because of the presence of pronounced geological heterogeneity and changing raypath orientations due to source-receiver configurations and strong seismic velocity gradients. Christoffel velocity equations for anisotropic media indicate large variation in P- and S-velocities can occur as functions of off-axis propagation angle. As a result, calculations of anisotropic effects are not simple in areas of complex crystalline crust and in some cases may only be solved numerically. Because we calculate multi-component seismograms within crustal anisotropic media using finite difference methods, we allow for full 3D variability in severity and orientation (tilt) of anisotropic material properties as expressed in composition, large-scale features, and internal textural fabrics. The full elastic stiffness tensor is used in the finite difference formulation; as a result, tilted hexagonal, orthorhombic, and lower order symmetries are possible. We present modeling theory plus 3-component synthetic seismograms from wave propagation within selected geometrical models of crustal features. We address two primary issues: (1) is the anisotropy observable within the seismograms, and (2) in sufficiently dense "arrays" are there systematics in the P- and/or S-wave phases which can be used to characterize the crustal feature. The selected features are calibrated using petrophysically measured velocities and densities. In particular, the one or three diagonal VP measurements (for hexagonal and orthorhombic symmetries) affect traveltime and shear wave splitting, as we confirm with modeling examples. We also discuss relationships between detectable anisotropy effects and internal configurations of the crustal features.