Computation of the Compressional Wave (Vp) to Shear Wave (Vs) Velocity Ratio from the 2018 Mpape Earth Tremor in Nigeria
Umar A. Kadiri *
Centre for Geodesy and Geodynamics, National Space Research and Development Agency, Toro, Bauchi State, Nigeria and Nnamdi Azikiwe University, Awka, Anambra State, Nigeria.
Maureen C. Umeh
Nnamdi Azikiwe University, Awka, Anambra State, Nigeria.
Ruth O. Ikeze
Nnamdi Azikiwe University, Awka, Anambra State, Nigeria.
*Author to whom correspondence should be addressed.
Abstract
Although Nigeria is generally considered to be located on a seismically stable part of the African plate, several earth tremors have been reported within the country, including the September 2018 Mpape event in the Federal Capital Territory, Abuja. The causes of these tremors and the related crustal and upper-mantle structure remain insufficiently constrained. This study computed the ratio of compressional wave velocity (Vp) to shear wave velocity (Vs) for the 2018 Mpape main tremor using available P- and S-wave arrival times from Nigerian seismic stations. The arrival times of regional phases, including Pg, Sg, Pn, and Sn, were obtained with Seisan software. The S-P arrival-time differences were calculated and plotted against the absolute P-wave arrival times using the Wadati-diagram approach. The Vp/Vs ratio was derived from the slope of the line of best fit using the least-squares method. The computed Vp/Vs ratio for the Mpape earth tremor was 1.76, with a root mean square value of 0.25 and a correlation coefficient of 0.999. The result suggests a relatively uniform lithology around the tremor epicentre and is consistent with the geological interpretation of the study area. The findings provide locally derived seismic parameters that may support future investigations of crustal structure, local earthquake location, and seismic-hazard assessment in Nigeria.
Keywords: Mpape earth tremor, Vp/Vs ratio, compressional wave velocity, shear wave velocity, Wadati diagram, least-squares method, Seisan software, seismic stations, crustal structure, seismic hazard assessment.