The Compressional to Shear Wave Velocity Ratio of the Crust and the Uppermost Mantle of Nigeria

Monday A. Isogun, Kadiri U. Afegbua, Abdulhamid T. Nuhu, Muhammed J. Abba

Abstract: The compressional to shear wave velocity ratio (Vp/Vs) of the crust and the uppermost mantle of Nigeria have been determined using data on four local earthquakes recorded by Nigerian Network of Seismographic Stations (NNSS) between 2009 and 2016. We used the Wadati diagram to compute the Vp/Vs values. The time of first arrivals of P-wave and S-wave mainly Pg Sg, Pn and Sn were obtained and the time difference between the P- and S-waves time arrivals were computed. A graph of S-P time difference against the absolute P-wave arrival times for each earthquake was ploted. The Vp/Vs values were computed from the slope of line of best fit of the graph determined using least square method. The Vp/Vs values derived from the four earthquakes are 1.756, 1.755, 1.760 and 1.758. The average of the different velocity ratios is 1.76 and this is considered to be the Vp/Vs ratio of the crust and the uppermost mantle of Nigeria. The result obtained however is subject to review with more seismic stations and local earthquakes within the region.

Keywords: Vp/Vs values, P-wave, S-wave, first arrivals, local earthquakes