Ambient Seismic Noise Tomography for Upper Crustal Imaging of the West African Craton in Nigeria

This study applies ambient seismic noise tomography to image upper crustal velocity structure across the West African craton in Nigeria, providing an original contribution to the geophysical characterisation of one of Africa's largest and oldest geological domains. The West African craton in Nigeria encompasses Archean and Proterozoic basement rocks in an area of 280,000 square kilometres whose upper crustal seismic velocity structure has never been directly imaged through tomography. Upper crustal velocity models are needed for improving earthquake hypocenter relocation, understanding the tectonic architecture of the craton, and developing seismic hazard assessments for Nigerian cities underlain by basement complex. This study installs a network of 35 broadband seismometers across Nigeria's basement complex terrain for 24 months of continuous recording. Interstation Green's functions are retrieved from cross-correlation of ambient noise wavefields. Rayleigh and Love wave group and phase velocity dispersion curves are extracted at 4 to 35 second periods. A 3D shear wave velocity model is constructed by simultaneous inversion of the surface wave dispersion maps using a finite-frequency tomographic kernel. Resolution analysis using synthetic checkerboard recovery tests determines the achievable model resolution. Findings reveal Rayleigh wave phase velocities that vary from 2.8 to 3.4 kilometres per second at 10-second period, with lowest velocities in the Niger Delta and Bida Basin and highest velocities in the Archean Jos Plateau basement. The tomographic model reveals a northwest-trending low-velocity anomaly in the upper 15 kilometres of the crust beneath the Auchi-Okene area that is interpreted as a Proterozoic mobile belt remnant not apparent in surface geological mapping. The study provides the first seismic noise tomographic model for Nigeria's basement complex.

Keywords: ambient seismic noise, tomography, West African craton, Nigeria, upper crustal velocity.