Attenuation tomography of the upper inner core
Date
2017
Authors
Pejic, Tanja
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Abstract
The Earth's inner core is one of the very few regions of our
planet that is still a puzzle. In order to understand planetary
formation and the geomagnetic
eld we need
to understand the structure and dynamics of the inner core.
Seismology provides us with some tools to probe into the core and
understand it better, for example,
body waves traveling through the deep Earth and interacting with
the propagating medium. In this study we use body waves,
speci
fically the PKIKP phase propagating
through the inner core and the PKPbc phase propagating through
the outer core to study the attenuation of seismic waves within
the inner core. Because these
two phases have similar paths through the lowermost mantle and
differ only within the core, the attenuation of the PKIKP phase
relative to PKPbc can tell us about
the regions of the inner core and the extent thereof of the
attenuation. This in turn sheds light on possible geodynamical
scenarios of the inner core, all of which are still subjects of
heated debates. Currently the widely accepted attenuation
structure of the inner core is of hemispherical nature with
quasi-eastern hemisphere more attenuating than the quasi-western
hemisphere, with two competing theories explaining this
observation. Using a full-waveform fi tting simulated annealing
algorithm we collect a dataset of t* parameter from 50 globally
distributed earthquakes from past two decades with magnitude
larger than 5.8. t* parameter is directly related to
quality factor Q { measure of attenuation in the medium. We
first perform a linearised inversion in which we assume that the
logarithm of inverse quality factor
Q is normally distributed. We connect the least-squares method
with probabilistic framework, and use optimisation techniques, in
order to get the maximum posterior
probability solution and its uncertainties. The inversion is
performed on a fi
xed, coarse grid (explicit parameterisation).
Regularisation, in form of damping,
and separately, smoothing, is used. The results of the inversion
and their robust estimate of uncertainty point to an attenuation
heterogeneity more complex than
the hemispherical structure. While the solution obtained through
linearised inversion is robust, imposing a regular global grid
and a
fixed number of parameters
in tomographic studies will often result in introducing artefacts
in regions of low ray coverage. Furthermore, explicit
regularisation methods are global in character,
making small-scale features hard to see or masking them
completely. This is why we also perform a transdimensional
Bayesian inversion for quality factor Q, in which the complexity
of the model is determined by the data and the data noise,
estimated in the inversion. Hence there is no need for a fixed
parameterisation or explicit
regularisation. A more realistic estimate of the uncertainties of
the solution is an added bonus of Bayesian inversion. The results
of this inversion are in agreement
with the linearised one and point to an attenuation structure
more complex than the hemispherical one. As such these results
give more weight to the models that
connect the dynamics of the inner core with the heat flow in the
lowermost mantle.
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Keywords
seismology, inner core, attenuation, tomography, deep earth, inversion, Bayesian
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