Investigation of the Crust and Upper Mantle Structures from Teleseismic P-Wave Travel Time Residuals Beneath some Seismic Stations in Nigeria

Description

– 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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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 (30^o-90^o) has average new residuals of 1.8 ± 0.3 s, Region II (210^o-270^o), 1.8 ± 0.3 s and Region III (270^o-330^o) 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.

Introduction

1.1 Background of the Study

An earthquake is the result of sudden release of energy in the earth’s crust that creates seismic wave. Everyday about fifty or more of such events occur worldwide, with the magnitudes which are strong enough to be recorded in Nigeria.

Each earthquake event radiates seismic waves which travel throughout the earth, some of these waves are readily detected with modern instrument (seismometer) stationed anywhere on the globe.

Depending on the distance of the earthquake focus from the recording station, the recorded seismic events are classified as local, regional or teleseismic event (Lay and Wallace, 1995).

The branch of science which deals with the study of generation, propagation and recording of elastic waves through the earth and of the sources that produce them is known as seismology. Seismological procedures provide the highest resolution of internal earth structure of any geophysical method.

This is because elastic waves have the shortest wavelength of any geophysical wave and the physics that governs them localizes their sensitivity spatially and temporarily to the precise path travelled by the energy (Lay and Wallace, 1995).

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