Surface Wave Dispresion between North and East Africa
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Date
1983-06
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Addis Ababa University
Abstract
A comprehensive study of dispersion of surface Rayleigh waves
for three paths: Gulf of Aden - Addis Ababa (GOA - AAE), Red Sea
Addis Ababa (RES - AAE) and Arab republic of Egypt - Addis Ababa
(ARE - AAE), lying between north and east Africa is made. Earlier
studies of surface wave dispersion in the region were made for short
epicentral distances without considering contribution by different
modes. The problem of contribution by different modes to dispersion
is inherent at short epicentral distance ranges.
Rayleigh wave trains identified from seismograms of Addis
Ababa seismic station (AAE) are enlarged and digitized. The results
are analysed using fixed window Fourier analysis and moving window
analysis (Landisman et a1. 1969). These methods have advantages in
displaying different modes which otherwise would be absent if the
usual peak and trough method is used.
Dispersion curves obtained by the above methods and the usual
peak and trough method are compared with theoretical dispersion
curves generated for different crustal models. An efficient
algorithm developed for solving the period equation on a computer
(Dunkin 1965; Robert H. Herrman personal communication 1980 ) is
used to obtain theoretical dispersion curves up to ten modes.
However, in this study the fundamental and first modes were found
adequate to explain the seismogram observations. A total of 150
theoretical models each with multimode outputs were considered.
A crustal thickness of 27 km with high Poisson's ratio overlying
an anomalous mantle (,d-- = 7.2 km/sec) 4 km thick is found for
GOA - AAE path. For RES - AAE and ARE - AAE paths crustal thicknesses
of 37 km and 38 km respectively overlying a normal mantle
are found. The Poisson's ratios for these paths are similar to
that of a 'normal continental crust.
Regarding resolution of modes for the period range considered
in this study both moving and fixed window methods give similar
results indicating that the former method is superfluous
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Keywords
Surface Wave Dispresion