THE CRUSTAL AND THE UPPER MANTLE STRUCTURE OF SOUTHWESTERN NIGERIA FROM INVERSION OF TELESEISMIC SURFACE WAVES

This work presents the first attempt at providing information on the shear-wave velocity structure of the crust and the upper mantle of southwestern Nigeria. In this study, the depth distribution of seismic wave velocities over southwestern Nigeria was determined using teleseismic surface waves of different azimuths recorded by our network using 3-component broadband single-station between November 2009 and 2011. The group velocities of Rayleigh and Love waves with surface wave magnitude ≥ 6.0 in the period range of 20 – 100 s were inverted for plane-layered earth shear-velocity structure. The shear-wave velocity of the crust increases gradually from about 3.12 to 4.29 km/s. The Moho was found at depth 40 km. The upper mantle shear-velocity increases from about 4.37 to 4.5 km/s between depth 40 and 73 km and a low velocity layer between depth 73 km and 278 km with shear-wave velocity 4.01 – 4.47 km/s. An average values for crust and upper mantle velocities have been assumed over southwestern Nigeria from seismic data of a single seismic station within this region, however, further studies on data from many stations when available in this area will help to improve on the result of this study.