Recent global travel time tomography studies by Zhou [1996]
and Van der Hilst et al. [1997] have been performed with cell
parameterizations on the order of those frequently used in regional
tomography studies (i.e. with cell sizes of 1-2 degrees). These new
global models constitute a considerable improvement of previous
results that were obtained with rather coarse parameterizations
(5 degree cells). The inferred structures are however of larger scale
than is usually obtained in regional models and it is not clear where
and if individual cells are actually resolved. Furthermore,
these global models have been constructed from a linearized
inversion in which the bending of ray paths due to lateral
heterogeneity has not been taken into account. This may, however,
be very important when imaging small-scale structure.
We have first performed a linear inversion in which we aimed at resolving
lateral heterogeneity on a smallest
scale of 0.6 degrees in the upper mantle and of approximately 1.2-3
degrees in the lower mantle. This allowed for the adequate mapping of
expected small-scale structures induced by, e.g. lithosphere
subduction and hotspots. To arrive at this, for global tomography, very
detailed image we employed an
irregular grid of non-overlapping cells adapted to the heterogeneous
sampling of the Earth's mantle by seismic waves. Furthermore, we exploited
a
totally reprocessed version of the global data set of the International
Seismological Center.
We will present the important features of the linear inversion,
which are: 100-200 km thin high velocity
slabs beneath all major subduction zones, sometimes flattening in the
transition zone, sometimes directly penetrating into the lower mantle;
large high velocity anomalies in the lower mantle that have been
attributed by Van der Hilst et al. [1997] and Grand et al. [1997] to
subduction of the Tethys ocean and the Farallon plate; low velocity
plumes continuing across the 660 km discontinuity to hotspots at the
surface under Iceland, East Africa, the Canaries, Yellowstone, and the
Society Islands.
The linear inversion can be regarded as the first step of a nonlinear
inversion scheme which is set up as a series of linear inversions and ray
tracing steps. In each ray tracing step the entire data set (approximately
5 million rays) needs to be raytraced through the updated velocity
model, which leads to new travel-times and ray paths that can be
used for the next
linear inversion.
First
results of this nonlinear inversion will be presented as well and we will
investigate the importance of
ray bending in global travel-time tomography.