Seismic Tomography and Crustal Velocity Structure Beneath the Nsukka Platform Using Local Earthquake Data

The crustal velocity structure beneath the Nsukka Platform in southeastern Nigeria remains poorly resolved, limiting our understanding of the tectonic setting and seismic hazard potential of the region. This study applies local earthquake tomography to image the P-wave and S-wave velocity structure of the crust beneath the Nsukka Platform using data recorded at the University of Nigeria Seismic Network. Arrival time data from 186 local events recorded during a 12-month monitoring period were used as input for the SIMULPS14 tomographic inversion algorithm. The resulting velocity models are expressed as perturbations from a one-dimensional reference model derived from previous controlled-source studies. P-wave velocity anomalies range from negative 5 to positive 7 percent relative to the reference model. High-velocity anomalies in the upper crust beneath the Nsukka area are interpreted as Precambrian basement highs or competent Cretaceous carbonate units. A low-velocity zone at depths of 8 to 14 km beneath the Anambra Basin sector is consistent with the presence of overpressured shales or partially saturated fractured zones within the sedimentary succession. Vp/Vs ratio anomalies provide additional constraint on fluid saturation patterns within the upper crustal volume. The tomographic images reveal a northeast-trending velocity discontinuity that is interpreted as a lithological boundary between the Nsukka Platform and the Anambra Basin subsurface. These results significantly advance the understanding of crustal structure in the region and provide a reference model for earthquake location and ground motion simulation.

Keywords: seismic tomography, P-wave velocity, Nsukka Platform, crustal structure, local earthquake