Geophysical investigation of Obot Ekpo Landslide site, Cross River State, Nigeria

• Electrical and seismic methods were used to study the cause of a landslide in Nigeria.
• Results show that impermeable shales and marls dominate the shallow subsurface.
• Impaired percolation at interface causes rock weakness and instability to develop.
• Mechanical transmission of wind forces via tree roots triggers the landslide.

Data from 2-D electrical resistivity tomography (ERT), vertical electrical sounding (VES) and seismic refraction were used to investigate the causes of the Obot Ekpo Landslide in the Odukpani Local Government Area (LGA), Nigeria. Results show that low resistivity shaly and/or clayey materials with bulk resistivity less than 10 Ω m dominate the shallow subsurface of the area. These argillaceous materials intercalate with marls (and/or mudstone/limestone) materials that are characterised by relatively higher resistivity values. Average P-wave velocities were observed to vary from 655 to 1381 and 1787 to 1820 m/s for the first and second layers respectively. The marls are laterally heterogeneous due to multiple fracturing at different locations caused by the probing action of tree roots in a sloping terrain and other mechanical stresses such as those set up by the expansion of clays in confined areas. Most of the downward percolating water gets into deeper layers through the fractures and at depths, are blocked by the underlying impermeable materials. As the blocked groundwater accumulates gradually at the interface of the marl and the underlying shale units, pore-pressure and uplift forces begin to increase while cohesive force is reduced, leading to instability. With increased rainfall, the ground becomes over-saturated and therefore the instability condition increases in the area. Transmission of mechanical forces of wind into the sloping ground through tree roots and to a lesser extent, mechanical vibrations from heavy-duty equipment working in the vicinity, caused the over-saturated materials to flow, leading to the landslide.