INVESTIGATION OF THE CRUST AND UPPER MANTLE STRUCTURES FROM TELESEISMIC P-WAVE TRAVEL TIME RESIDUALS BENEATH SOME SEISMIC STATIONS IN NIGERIA
INVESTIGATION OF THE CRUST AND UPPER MANTLE STRUCTURES FROM TELESEISMIC P-WAVE TRAVEL TIME RESIDUALS BENEATH SOME SEISMIC STATIONS IN NIGERIA
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Date
2014-05
Authors
YAKUBU, TAHIR ABUBAKAR,
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Abstract
The crust and upper mantle structure beneath some seismic stations in Nigeria from the study of teleseismic P-wave travel time residuals has been investigated with the aim of determining the velocity structure within the crust and the upper mantle. This was achieved by using Seismic data from three stations at Ile-Ife, Nsukka and Kaduna which were consistent with data from July 2009 to July 2011. Five hundred and sixty-six (566) events were recorded: Ile-Ife, 109 events, Nsukka, 240 events and Kaduna, 217 events. The travel time plot for each of the three stations shows correlation with the standard travel time plot. The travel time residuals calculated with respect to the International Association for Seismological Practices 91(IASP91) model for each of the stations ranges from -0.52 s to 4.93 s with an average value of 1.8 ± 1.3 s for Ile-Ife station, -1.15 s to 4.4 s, with an average of 1.9 ± 1.4 s for Nsukka station and from -0.35 s to 4.86 s with an average 2.2 ± 1.3 s for Kaduna station. The large standard deviation in the average residuals is accounted for by considering the heterogeneity in the travel paths of the seismic waves to the recording stations. Therefore, to ensure that the data belong to a fairly homogeneous path, the events were divided into three azimuthal regions where the events are more concentrated and new residuals for each region were calculated for each station: For Ile-Ife station Region I (30o-90o) has average new residuals of 1.8 ± 0.3 s, Region II (210o-270o), 1.8 ± 0.3 s and Region III (270o-330o) 1.5 ± 0.4 s; For Nsukka station, Region I has new average residuals of 1.83 ± 0.9 s; Region II, 1.90 ± 0.5 s and Region III 1.5 ± 0.7 s while for Kaduna station, we have Region I, 2.2 ± 0.6 s; Region II, 2.2 ± 0.5 s and Region III, 1.7 ± 0.6 s. The new station residuals show less scatter hence the standard deviations are lower because the data sets now belong to fairly homogeneous travel paths. The positive residual
observed for all the stations is an indication of the presence of a low velocity structure within the crust and the upper mantle along the propagation path. Azimuthal variations of the residuals also show that the velocity structure along the regions I and II are thicker than in region III. This is accounted for by low value of the residuals compared with the total event residuals.
In other to remove the earthquake source and travel path effects from the travel time residuals, relative travel time residuals were computed for each of the three stations under considerations. The results shows that IFE and NSUKKA stations have negative (-0.03 s and -0.13 s respectively) relative travel time residuals, while KADUNA station has positive (0.15 s) relative travel time residual. Inversion of the travel time data reveals a velocity variation between 8.5 and 12.8 km/s indicating that the waves travel mainly in the mantle. The plot of the velocity-depth model for the three stations revealed a three layer mantle model, with transition at an average depth of 720 km.
Description
A DISSERTATION SUBMITTED TO THE POSTGRADUATE SCHOOL, AHMADU BELLO UNIVERSITY, ZARIA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR AWARD OF THE DEGREE OF DOCTOR OF PHILOSOPHY IN APPLIED GEOPHYSICS
DEPARTMENT OF PHYSICS
FACULTY OF SCIENCE
AHMADU BELLO UNIVERSITY
ZARIA, NIGERIA
MAY, 2014
Keywords
INVESTIGATION,, CRUST,, MANTLE,, STRUCTURES,, TELESEISMIC,, P-WAVE,, RESIDUALS,, SEISMIC,, STATIONS,, NIGERIA