Article,
Monte-Carlo inversion for a global shear-velocity model of the crust and upper mantle
Geophysical Journal International, 151 (1): 88--105 (October 2002)
DOI: 10.1046/j.1365-246X.2002.01742.x
Abstract
We describe a method to invert surface wave dispersion data for a
model of shear velocities with uncertainties in the crust and uppermost
mantle. The inversion is a multistep process, constrained by a priori
information, that culminates in a Markov-chain Monte-Carlo sampling
of model space to yield an ensemble of acceptable models at each
spatial node. The model is radially anisotropic in the uppermost
mantle to an average depth of about 200 km and is isotropic elsewhere.
The method is applied on a 2 x 2 deg grid globally to a large data
set of fundamental mode surface wave group and phase velocities (Rayleigh
group velocity, 16-200 s; Love group velocity, 16-150 s; Rayleigh
and Love phase velocity, 40-150 s). The middle of the ensemble (Median
Model) defines the estimated model and the half-width of the corridor
of models provides the uncertainty estimate. Uncertainty estimates
allow the identification of the robust features of the model which,
typically, persist only to depths of \~250 km. We refer to the
features that appear in every member of the ensemble of acceptable
models as 'persistent'. Persistent features include sharper images
of the variation of oceanic lithosphere and asthenosphere with age,
continental roots, extensional tectonic features in the upper mantle,
the shallow parts of subducted lithosphere, and improved resolution
of radial anisotropy. In particular, we find no compelling evidence
for 'negative anisotropy' (Vsv > Vsh) anywhere in the world's lithosphere.
