HR: 0830h
AN: T41C-0227    [PDF]
TI: Azimuthal Anisotropy Across the Pacific
AU: * Smith, D B
EM: dansmith@colorado.edu
AF: Department of Physics, University of Colorado at Boulder, Campus Box 390, Boulder, CO 80309-0390 United States
AU: Ritzwoller, M H
EM: ritzwoller@ciei.colorado.edu
AF: Department of Physics, University of Colorado at Boulder, Campus Box 390, Boulder, CO 80309-0390 United States
AU: Shaprio, N M
EM: shaprio@ciei.colorado.edu
AF: Department of Physics, University of Colorado at Boulder, Campus Box 390, Boulder, CO 80309-0390 United States
AB: We present maps of the $2\psi$ component of azimuthal anisotropy across the Pacific Ocean for Rayleigh wave group velocities at periods ranging from 20 sec - 150 sec. The fast axis directions and relative amplitudes at large scales ($>$ 1000 km) across the Pacific are robust to ad-hoc choices of smoothing and damping, but the absolute amplitude of azimuthal anisotropy remains poorly determined. At long periods (100 - 150 sec), anisotropic fast axes align approximately parallel to current plate motions, but at short periods (20 - 50 sec) fast axes diverge from current plate motion directions in the western Pacific and align more closely with the local paleo-spreading direction. This behavior provides evidence that anisotropic fabric in the deep lithosphere and asthenosphere evolves with the plate and is able to conform with contemporary plate motions. In the shallow lithosphere, however, anisotropic fabric appears to be ``frozen in'' and remains representative of the conditions at the time of formation of the lithosphere. In addition, the amplitudes of azimuthal anisotropy decrease with lithospheric age at periods between 50 and 150 sec. This may be the effect of a process or processes that disrupt azimuthal anisotropy in the central Pacific, such as plumes or lithospheric instabilities (small-scale convection).
DE: 7218 Lithosphere and upper mantle
DE: 7255 Surface waves and free oscillations
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Tectonophysics [T]
MN: 2003 Fall Meeting