2.2 $s$-wave tomography

grand:

equal-area block model as of Grand's ftp-site in fall 2000 [see Grand et al., 1997], distributed on a $2^\circ\times 2^\circ$ grid. The model was derived from a combination of body and surface wave measurements with a two step process [ Grand, 1994]: first, observations are explained in terms of upper and lowermost mantle structure only. Second, the authors invert the residual travel-time anomalies to find velocity heterogeneities in the rest of the mantle.

ngrand:

Updated version of grand, as of Grand's ftp-site in June 2001. The inversion that led to ngrand was damped more strongly in the upper mantle than that of grand (S. Grand, pers. comm.); as a result, the new model is different from grand mostly in amplitude, rather than pattern, of heterogeneity (see the G-Cubed article ).

s20rts:

Caltech model S20RTS, parameterized horizontally in terms of spherical harmonics up to degree $\ell_{\mathrm{max}}=20$, and radially with a set of cubic splines [ Ritsema and van Heijst, 2000]. Derived from a data set that, in addition to body and Rayleigh wave measurements, includes observations of normal mode splitting functions.

saw24b16:

Berkeley $v_{sh}$ model SAW24B16 [ Mégnin and Romanowicz, 2000], derived by fitting body and surface wave transverse-component waveforms. Parameterized with spherical harmonics ( $\ell_{\mathrm{max}}=24$) and cubic splines.

sb4l18:

Scripps model SB4L18, from observations of body, Love, and Rayleigh waves, and normal modes [ Masters et al., 1999]. Parameterized in terms of equal-area blocks ( $4^\circ\times 4^\circ$ at the equator) with 18 radial layers.

s20a:

Harvard model S20A from observations of body, Love, and Rayleigh waves [ Ekström and Dziewonski, 1998]. $v_{sh}$ and $v_{sv}$ anomalies were treated as independent free parameters; $v_s$ is subsequently estimated from their Voigt average. Spherical harmonics ( $\ell_{\mathrm{max}}=20$) horizontally; radially, upper and lower mantle are parameterized separately with two sets of Chebyshev polynomials.

s362d1:

Harvard model S362D1, derived with a procedure analogous to s20a (including the discontinuity at 660-km) but described by a cubic spline parameterization both horizontally and vertically [ Gu et al., 2001]. Lateral resolution is equivalent to $\ell_{\mathrm{max}}\sim18$.

In addition, we will also use lower resolution joint inversions for $v_s$ and $v_p$. We consider MK12WM13 [ Su and Dziewonski, 1997] (spherical harmonics, $\ell_{\mathrm{max}}=12$, Chebyshev polynomials with depth, $v_p$ and $v_s$ anomalies denoted by mk12wm13p and mk12wm13s, respectively), SB10L18 by Masters et al. [2000] (similar to sb4l18 but 10 $^\circ \times 10^\circ$ blocks, pb10l18 and sb10l18), and Harvard model SPRD6 from normal mode splitting coefficients [ Ishii and Tromp, 2001] (spherical harmonics, $\ell_{\mathrm{max}}=6$, sprd6p and sprd6s).