The Upper Mantle Seismic Velocity Structure of South-Central Africa and the Seismic Architecture of Precambrian Lithosphere Beneath the Congo Basin - Geology and Resource Potential of the Congo Basin - page 6

1.4.1 Rayleigh Wave Group Velocity Maps

open data from permanent and temporary networks, as well
as from many new temporary and permanent seismic
stations in eastern and southern Africa that are part of
AfricaArray ( http://www.africaarray.org ) (Fig. 1.3 ). Tempo-
rary stations were deployed in Uganda, Tanzania and
Zambia between 2007 and 2011 as part of the AfricaArray
East African Seismic Experiment (AAEASE) and the
AfricaArray Tanzania Basin Seismic Experiment
(AATBSE) (Tugume et al. 2012 ; Mulibo and Nyblade
2013 ), and several permanent stations have been established
since 2005 to form the AfricaArray observatory network.
Data from the AfricaArray stations greatly improves ray
path coverage across the Congo Basin. In our modeling
approach, maps of Rayleigh wave group velocities are first
constructed using event-station measurements of group
velocities from fundamental mode surface waves, dispersion
curves are extracted over a grid of nodes and inverted for 1D
shear velocity models, and then the 1D models are combined
to create a quasi-3D upper mantle shear wave velocity model
of the region.

To develop the group velocity maps, we used seismic events
with magnitude greater than 4.5 and depth shallower than
100 km recorded between 1990 and 2013. Only events along
the margin or within the African plate have been used to
minimize the influence of structure outside the plate on our
tomographic images.
Group velocity measurements for 40,337 event-station
pairs were made for fundamental mode Rayleigh waves
using vertical component seismograms. An example is
shown in Fig. 1.4 for a magnitude 5.5 event recorded on station
KMBO. The ray path coverage is illustrated in Fig. 1.5 .After
visual inspection for waveform quality, the instrument-
corrected data were windowed to isolate the Rayleigh wave,
and group velocity measurements were made using the multi-
ple filter method (Dziewonski et al. 1969 ; Herrmann 1973 )
implemented in the program PGSWMFA (PGplot Surface
Wave Multiple Filter Analysis; Ammon 1998 ). An example
measurement is shown in Fig. 1.6 for the same data as in
Fig. 1.4 .

Fig. 1.3 Locations of seismic stations used for this study. Color-coded FDSN network codes are provided at the bottom of the figure