Up to now tomographic travel-time studies of the global seismic
velocity structure have been hampered by insufficient data quality
and by the basic linearization of the nonlinear travel-time problem
about simple 1-D reference models of the Earth. The availability of
a new and accurate data set of global first and later seismic
arrivals, and the development of new 3-D ray tracing techniques,
enable for the first time to assess the global nonlinear travel-time
problem. The main research goal is to estimate a solution to the
global nonlinear travel-time problem by accounting for the effects
of 3-D heterogeneity on ray bending, travel times of seismic phases
and earthquake locations.
The entire research project basically consists of three parts. In
part 1 the new, global data set of Engdahl et al. (1998) was screened
and filtered. Nearly 8 million P and pP phases were selected for the
first inversion step: the linear inversion. From the linear inversion
a high resolution (smallest cellsize = 60 km) global model
was obtained, which
accurately depicts the major tectonic structures of the Earth in regions
with sufficient data sampling. Most prominent are
the high velocity anomalies
associated with subduction zones, which often seem to extend well into
the lower mantle.
Part 2 consisted of the extension and comparison of 2 existing ray
tracing methods for their application in the global, linear model.
The main task was the construction of a very fast ray tracer to
calculate the 8 million, bent ray paths in a reasonable amount of time.
In part 3 a second inversion step has been performed, which completes the
nonlinear inversion. The resulting model has been compared with the linear
model to assess the necessity of nonlinear inversion for global tomography.
The high resolution global model may be used for comparison with tectonic
reconstructions and for other global applications, e.g. related to
mantle convection
or earthquake location.
Techniques: Large, sparse matrix inversion, staggered grids
and ray tracing.
Duration: 01-12-94 until 08-11-99.