Magnitude and symmetry of seismic anisotropy as a constraint on crustal composition and structure

Our ability to assess seismic hazard depends in large part on our understanding of the composition and structure of the crust, of which only the upper third can be directly sampled. For the middle and lower crust we must rely on our ability to interpret geophysical observations, including seismic anisotropy, to better understand composition and structure. Current methods of measuring crustal anisotropy use the assumption of transverse isotropic symmetry. While complicated, the symmetry of elastic properties of crustal materials has the potential to be used to help differentiate between materials of different composition. We present results of calculations of elastic tensors based on electron backscatter diffraction (EBSD) measurements of mineral crystallographic preferred orientations (CPO) in crustal rocks from a number of locations, including new results from the Appalachian Mountains. Elastic tensors can be deconvolved into their main symmetry components to illustrate the importance of low-order symmetry in constraining rock type and composition. Initial results indicate that all crustal rock types with at least 5% anisotropy have a significant component of transverse isotropic (TI) symmetry, but many also have a significant component of orthorhombic symmetry. TI and orthorhombic components appear to be related to amounts of mica and amphibole, respectively. Because seismic methods assume TI symmetry, misfit in seismic inversion may in fact be a useful indicator of composition.