CRUSTAL SEISMIC ANISOTROPY - SOME FIRST PRINCIPLES

Seismic anisotropy is the cumulative interplay between propagating elastic waves and anisotropic earth material. The unraveling of this effect in deformed crustal terranes is complex due to geologic 3D geometry and heterogeneity, the bending of seismic raypaths due to velocity gradients, and often the observation of anisotropy as second-order waveform/traveltime effects. Both the seismic wave and crustal structures provide factors that influence the production of the seismic response. Four characteristics of the seismic wave are important: (1) path length, the amount of exposure to the anisotropic material, (2) raypath direction, relative to the rock anisotropic properties, (3) polarization, the type of wave and its vibration relative to the media, and (4) wavelength, affecting the scale of structures to which it may respond.

While seismologists recognize that seismic anisotropy can originate from upper crustal fractures or by organized fine-scale layering of isotropic material, here we are most interested in crustal anisotropy produced by deformation during regional metamorphism. Fabric material anisotropy involves at least four factors that contribute to seismic anisotropy: (1) mineral CPO (crystal preferred orientation) and rock texture, (2) bulk representation and scales, (3) structural shape and internal geometries (e.g., "structural geometry" anisotropy), and (4) azimuthal behavior of the anisotropy and the necessity to not use the weak-anisotropy assumption. Recent laboratory measurements of rock velocities reveal that a "diagonal quasi-compressional wave measurement" or "VP-45^o" effect can drastically alter the azimuthal behavior of seismic anisotropy in foliated crustal rocks.

Seismic anisotropy is best understood using the elastic stress-strain definition of wave propagation. Here, the earth media are represented by stiffness tensors. The anisotropic behavior of fabrics can be classified by their tensor symmetries (e.g., isotropic, hexagonal). In addition, tensor averaging methods allow us to examine the anisotropic response of larger scale structures occurring within metamorphic terranes which possess strong coherent internal structure. We review these factors that affect seismic anisotropy and identify topics that require further community investigations.