HR: 13:55h
AN: T52E-02    [PDF]
TI: Cause and Effect: Seismic Anisotropy Measurements Using Full Wavefield Simulations in Realistic Mantle Flow Models.
AU: * Levin, V
EM: vadim@ldeo.columbia.edu
AF: Dept. of Geological Sciences, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854 United States
AU: Okaya, D
EM: okaya@usc.edu
AF: Dept. Earth Sciences, Univ. Southern California, Los Angeles, CA 90089-0740 United States
AB: The supra-slab mantle wedge is a site of relatively small-scale flow of mantle material. Wedge flow is controlled by the rate, angle and direction of slab descent, the shape of the subducting slab and the nature and behavior of the overriding plate. The simplest geodynamic model for the mantle wedge is a 2-D corner flow driven by the shear coupling to the down-going plate. An upper mantle wedge at a convergent margin that deforms in corner flow should develop a trench-normal olivine LPO texture detectable by seismic anisotropy studies. It is considerable concern therefore that most studies of seismic anisotropy within supraslab wedges do not conform to expectations from this simple 2-D corner flow regime. We perform a systematic exploration of seismic anisotropy indicators expected in a subduction zone. Our primary goal is to separate effects of "true" anisotropy from other contributions to the complexity of S waveforms, like propagation in complex 3D structure, specific biases of individual measurement techniques, frequency content of the wavefields etc. We use models composed of the mantle, the slab, the supraslab wedge and the crust. Anisotropy may be prescribed in one or more regions of the model. In particular, in the supraslab wedge we prescribe flow lines, and an associated pattern of anisotropic seismic velocities. Our wave propagation code allows for 3D heterogeneity in (anisotropic) velocity structure (crust, slab, subslab mantle, supraslab mantle) plus tilt for alignment with flow fields of any geometry. The code uses a finite difference solution of the elastic wave equation and allows for both point sources and upcoming plane waves of any back-azimuth and inclination (Okaya and McEvily, 2003). We compute S wavefields for a range of incidence parameters, and perform measurements of anisotropy as we would if these were real data. To evaluate resulting birefringence, we use two commonly used techniques for estimating shear wave splitting. The first method seeks a rotation of observed seismograms that would yield waveforms on two components that are most similar, and then evaluate the delay between them via cross-correlation. The second method seeks a combination of a rotation and a delay which, once it is used to correct for the effect of anisotropy, would minimize one of the components of motion. We also use a recently developed algorithm of Menke and Levin (2003) that is based on explicit wavefield comparison of "observed" and "predicted" wavefields, and allows to search for group solutions using multiple observations. Preliminary results show that the combined influence of effects other then anisotropy cause significant "deviations" of seismic anisotropy parameters recovered by standard measurement techniques from the "true" flow patterns in the model mantle. Of particular importance is the geometry of the wave propagation relative to the flow pattern.
DE: 7203 Body wave propagation
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Tectonophysics [T]
MN: 2003 Fall Meeting