A DETAILED STUDY OF TELESEISMIC P-WAVE TRAVEL TIME RESIDUALS AT AHMADU BELLO UNIVERSITY ZARIA SEISMIC STATION
A DETAILED STUDY OF TELESEISMIC P-WAVE TRAVEL TIME RESIDUALS AT AHMADU BELLO UNIVERSITY ZARIA SEISMIC STATION
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Date
1999-08
Authors
ONIKU, ADETOLA SUNDAY
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Abstract
The teleseismic P-wave travel time residuals at the Ahmadu Bello University
(A.B.U.) Zaria seismic station were measured with respect to the Jeffery-Bullen (JB)
tables. A total of two hundred and thirty (230) events at epicentral distances
between 24° and 180° was selected from recordings covering the period January
1985 to December, 1992. The limited number of events used was due to the low
operational gain (due to high cultural background noise) at the station which
permitted only events whose magnitudes are large enough to be picked. This was
further reduced to 108 events which have residuals less or equal to 5.
The data obtained were used to create the station history plots which display
the travel-times on the distance-time curve, event locations on a world map
centered at Zaria and the P-wave residuals for the station.
The station absolute residuals for all the selected events vary between -4.1
s to 5.0 s with an average value of 1.08 s ± 2.0 s. The azimuthal biases were then
taken into consideration by sorting the residuals into 10° azimuth by 10° distance
bins. A new station average residual was calculated to be 0.72 ± 1.81 s by summing
over the average residual for that bin. These are then referred to as the residuals
before and after sorting respectively. The large standard deviation of the average
residuals suggests that the variation in the residuals cannot be accounted for by
considering the data as belonging to one homogeneous set. The data set was
therefore sub-divided into five regions on the basis of similar azimuths and
propagation path to the station. The average residuals for each of the five regions
are:
(i) Mediterranian +1.05±0.85s
(ii) Kazakhstan -0.90+0.40 s
(iii) Burma/lndia/China +2.33+1.18 s
(iv) South and Central America +1.40+1.00 s
(v) North Ascension Island +0.20+0.70 s
Relative travel time residuals of the station were calculated with reference to
some group of stations located in the stable West-African Craton, Congo Craton
and the seismically active East-African rift valley by considering some events
distributed randomly in the five different regions. It was found that the trend of the
relative residuals is an indication of lateral inhomogeneity of the crust and upper
mantle between the various stations and Zaria station. It was also found that the
relative residuals are highly dependent on the direction of approach of the P-waves.
An inversion of the travel time data was attempted using the tau-p method
of inversion. However the observed data were found to be too few and sparse in
any given azimuth to yield meaningful results.
Description
A RESEARCH WORK SUBMITTED TO THE DEPARTMENT OF PHYSICS
AHMADU BELLO UNIVERSITY, ZARIA IN PARTIAL FULFILLMENT FOR THE
AWARD OF M. Sc. DEGREE IN APPLIED GEOPHYSICS
Keywords
Detailed,, STUDY,, TELESEISMIC,, P-WAVE,, TRAVEL,, TIME,, RESIDUALS,, AHMADU BELLO UNIVERSITY ZARIA,, SEISMIC,, STATION