Geoscience Reference
In-Depth Information
11
Global Imaging of the Earth's
Deep Interior: Seismic Constraints
on (An)isotropy, Density
and Attenuation
JEANNOT TRAMPERT AND ANDREAS FICHTNER
Department of Earth Sciences, Utrecht University, Utrecht, The Netherlands
Summary
long-period (mode splitting) ends of the spectrum,
(2) the implementation of fully probabilistic ap-
proaches, (3) the joint inversion of phase and
amplitude information for elastic and anelastic
structure, (4) the design of misfit functionals tar-
geted at specific aspects of Earth structure, and (5)
the continuing development of full waveform in-
version techniques. Furthermore, much progress
can be made by objectively quantifying the uncer-
tainties of our inferences. This can lead to both
more consistent models of the Earth and more
reliable interpretations in terms of its thermo-
chemical structure and evolution.
Seismic tomography is our principal tool to probe
the deep interior of the Earth. Over the past
decades, it has drawn the picture of a vigor-
ously convecting planet, with large-scale up- and
down-wellings convincingly imaged by isotropic
velocity variations. Models of seismic anisotropy
induced by crystal alignment provide insight into
the underlying convective motion, and variations
of density allow us to discriminate between ther-
mal and compositional heterogeneities.
However, despite substantial progress, enor-
mous challenges remain: The strength of im-
aged anisotropy trades off nearly perfectly with
the roughness of isotropic heterogeneity, which
complicates any quantitative interpretation. Den-
sity variations in the Earth, except for spherical
harmonic degrees 2, 4 and 6, are still largely
unknown, and error estimates are dominated
by subjective regularization. Concerning attenua-
tion, any pair of global 3Dmodels is uncorrelated,
with mutual differences larger than the attributed
uncertainties.
Most of these difficulties can be traced to the
complicated nature of multi-observable/multi-
parameter inverse problems that suffer fromweak
constraints and complex trade-offs. Promising di-
rections to strengthen constraints and reduce
trade-offs, include (1) the addition of new data
at both the short-period (scattered waves) and
11.1 Introduction
Seismic tomography is central to the multi-
disciplinary effort aimed at understanding the
structure and thermo-chemical evolution of
the Earth. Indeed, as seismic waves produced
by earthquakes travel through the deep Earth,
they absorb information from the (an)elastic and
density structure along their way which can
be extracted by seismic imaging. Compared to
geological time scales, seismic waves record an
instantaneous snapshot of the current Earth,
although anisotropy provides some constraints
on geological events in the Earth's recent history.
The process of absorbing the information is a
well posed forward problem and currently, with
