Thorsten W. Becker

Jackson School of Geosciences
The University of Texas at Austin

Anisotropy model


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Upper mantle, seismic anisotropy from LPO and mantle flow model

Here are global plots and model data files from our seismic anistropy models for the upper mantle. Models are based on the formation of lattice preferred orientation (LPO) textures of olivine/enstatite during asthenospheric flow, and model I below was recently reevaluated in light of azimuthal anisotropy by Becker et al. (2014) and for radial anisotropy by Auer et al. (2015) (Additional notes.)
  1. Model I

    Best-fit model (safs417nc3_er) of Becker et al. (2008).
    • Velocity fields for the flow model

      We provide a low resolution, Netcdf/GMT grd representation of velocities of the actual flow model on which LPO computations are based in this gzipped tar file safs417nc3_er.vel.tgz. The files are v[rtp].i.grd for spherical coordinates r, theta, phi velocities (cm/yr) at i = 1, 2,...44 layers depth, sorted bottom up with depth in km given in the file vdepth.dat.
    • Horizontal projection of fast axes for azimuthal anisotropy/SKS

      SKS splitting, for example, results from a non-linear averaging over the anisotropic elasticity tensor along the path (e.g., Becker et al., 2012). However, simplified representations can be compared with azimuthal anisotropy estimates. Here, we use the best fit hexagonal approximation of the tensor (transverse isotropy, TI, computed as in Browaeys and Chevrot, 2004), projected into the horizontal, and expressed in %.
    • Full anisotropic elasticity tensors

      • Model data files are given at the same depth levels as above (50, 100, ..., 350 km):
        Data are in format:
        lon lat depth(km) ... 
        V_11 V_12 V_13 ... V_16 ...
        V_22 V_23 ... V_26 ...
        V_33 V_34 ... V_36 ...
        V_44 ... V46 V_55 V_56 V_66
        
        where V is the Voigt matrix representation of the elasticity tensor (in the convention of Browaeys and Chevrot, 2004, for example), given in upper triangle representation, units of GPa.

        The fstrack software package contains a number of program to process elasticity tensors in the sav format from above. For example, the best-fit TI axes were computed with sav2decompose.

    • Additional notes

      This particular LPO from flow model incorporates lateral viscosity variations in the mantle and uses dislocation/diffusion creep rheologies; it was found to match global, seismologically determined radial anisotropy averages by Becker et al. (2008), see, e.g., their Fig. 7c.

      The LPO model also matches seismologically imaged patterns of azimuthal and radial anisotropy well globally as inferred from large-scale surface-wave imaging. It can therefore in some sense be considered a reference model for the LPO-induced seismic anisotropy from mantle flow in the upper mantle (Becker et al., 2014, Auer et al., 2015), subject to further refinement.

      In particular, the flow model does not include smaller-scale density anomalies, such as underneath North America, or any intraplate plate deformation, or microplate motions, on scales smaller than the plates of the NUVEL model. This model also only captures the active mantle flow component of LPO anisotropy formation, and layers shallower than ~100 km will have little anisotropy in the model. Frozen-in anisotropy in reality may be important at those depths (Becker et al., 2008).

      The actual anisotropy is based on Kaminski et al.'s (2004) DREX method and assigning pressure and temperature dependent single crystal tensors before averaging (Becker et al., 2006), and therefore consists of the full, 21 independent elastic tensor components.

References


[news] [research] [teaching] [group] [publications] [CV] [contact]
downloads: [software] [tomography] [visualizations]
[global maps] [SKS compilation] [APM model] [LPO model]
[lecture notes] [papers]

Updated: December 13, 2018. (c) JSG Geodynamics, 2018.