Geology Reference
In-Depth Information
Fig. 1.4 Sample vertical component displacement seismogram for a magnitude 5.5 earthquake recorded on station KMBO shown with several
bandpass filters illustrating the dispersion of the Rayleigh wave. The instrument response has been removed from the seismogram
similar to the surrounding Precambrian terranes. It also does
not support the crustal thinning model of Kadima et al. ( 2011b ).
To estimate the uncertainties in our velocity model, we
follow the approach developed by Juli ` et al. ( 2005 ) for their
inversion method, which involves repeatedly performing the
inversions using a range of inversion parameters. This may
be viewed as an overly simplistic approach, but it is effective
in developing a sense of the range of variation in the inverted
parameters given the observations and apriori constraints.
As explained by Juli` et al. ( 2000 ), more sophisticated and
statistically rigorous approaches generally require that the
observations meet desirable properties, such as being nor-
mally distributed, which in general are not satisfied by the
data. By following the approach of Juli` et al. ( 2005 ), we
obtain an uncertainty of approximately 0.1-0.2 km/s for the
velocity in each model layer.
The results for upper mantle depths of 50 to 90 km are
shown in Fig. 1.11 . Shear wave velocity structure is less well
resolved at deeper depths. The shear wave velocity pattern is
similar to the group velocity pattern at periods of 60 s and
higher. There are several areas with faster velocities that
correspond to the Archean cratons (Bomu, Kaapvaal,
Kasai, Ntem, Tanzania, Zimbabwe) with slower velocities
marking Proterozoic mobile belts in between. The mobile
belts that exhibit slower upper mantle velocities include the
Mesoproterozoic Kibaran, Karagwe-Ankole, Irumide and
Southern Irumide Belts and the Neoproterozoic Damara
Belt (Figs. 1.1 and 1.11 ). The Paleoproterozoic Bangweulu
Block in northern Zambia also has lower upper mantle
velocities than the Archean cratons. Within the Congo
Shield (Fig. 1.1 ), the fastest velocities are found beneath
the southern and central portions of the Congo Basin. The
regions of fast velocity extending from coastal regions off-
shore Angola, Namibia and South Africa probably result
from smearing of upper mantle structure across the conti-
nent-ocean boundary. In the resolution tests (Fig. 1.10 ),
checkers in these regions are less well resolved than within
the interior of the continent. Similarly, the region of fast
upper mantle velocities in South Sudan probably results
from structure in that part of the model being less well
resolved than to the south.
1.5
Comparison of Models
We briefly compare the new model of uppermost mantle
structure at 90 km depth (Fig. 1.11 ) with the model from
Fishwick ( 2010 ) at 100 km illustrated in Fig. 1.2 . To first
order, the figures show similar structures. In both models the
upper mantle beneath the Congo Shield has faster velocities,
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