HR: 0830h
AN: S11C-0303    [PDF]
TI: One Plus two Equals Five: How Layering and Nonhexagonal Symmetry Mess With Crustal and Mantle Anisotropy Measurements
AU: Blackman, D K
EM: dblackman@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Ave A-0225, La Jolla, CA 92093 United States
AU: * Schulte-Pelkum, V
EM: vera_sp@cires.colorado.edu
AF: University of Colorado, UCB 399 2200 COlorado Ave, Boulder, CO 80309 United States
AB: The effects of layered anisotropy on seismic observations are not straightforward. We predict observations for crustal anisotropy using a reflectivity method for layers with general symmetry and orientation. Although $P$ delays do accrue arithmetically over the anisotropy along the path, shear wave splitting and $P$ polarization combine the encountered anisotropy in a nonlinear fashion. Such results are dependent on the wavelength of the signal relative to the layer thicknesses, and variations can be strong functions of station-event azimuth. If the standard single-layer splitting method is applied in the layered anisotropy case, the measured splitting time may be larger (or much less) than the sum of the splitting times of each layer. The measured fast azimuth may point away from the fast azimuths of the individual layers, rather than lie somewhere in between. While it is usually assumed that the crustal contribution to SKS splitting is negligible, we demonstrate that realistic crustal anisotropy can significantly alter the mantle signal in some cases. A second factor affecting anisotropy measurements is a possible oversimplification by assuming hexagonal symmetry (also called transverse isotropy or TI). Foliated crustal rocks such as gneisses and schists often exhibit orthorhombic symmetry, which leads to a bias in measurements when hexagonal symmetry is assumed. We present predictions for anisotropic observations from numerical modelling using crustal and mantle petrological tensors and demonstrate the effects of layering and symmetry class, and give bounds on the range of expected values based on crustal samples.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 7203 Body wave propagation
DE: 7260 Theory and modeling
DE: 8110 Continental tectonics--general (0905)
DE: 8164 Stresses--crust and lithosphere
SC: Seismology [S]
MN: 2003 Fall Meeting