Electrical Resistivity Tomography Investigation of Active Fault Traces in the Ado-Ekiti Area, South West Nigeria

Active faulting in the crystalline basement terrain of Ekiti State has been proposed as a contributing mechanism for the occurrence of minor seismic events and ground cracking features documented in Ado-Ekiti and surrounding communities, but the near-surface geometry and activity status of these faults are uncharacterised by direct geophysical investigation. This study employs electrical resistivity tomography to investigate the near-surface traces of proposed active faults in the Ado-Ekiti area and assess their geotechnical significance. Eight ERT profiles of 200 to 400 metres, each oriented to cross the inferred fault traces at high angles, were acquired using the Wenner-Schlumberger array with 5-metre electrode spacing for adequate near-surface resolution. Inverted resistivity models from profiles crossing one fault trend reveal a consistent pattern of low-resistivity zones between 5 and 25 metres depth that narrow toward the surface and are associated with clay gouge materials, consistent with a fault core. Lateral offsets in the depth to fresh basement of 3 to 8 metres are observed across the inferred fault traces. Comparison of ERT-defined fault trace positions with mapped surface crack lineaments confirms spatial coincidence to within 10 metres at two locations. A trench investigation at the most clearly defined ERT target exposed a 0.8-metre wide clay-gouge zone consistent with the geophysical model. Luminescence dating of sediment adjacent to the exposed fault scarp indicates the most recent displacement occurred approximately 3,500 years before present. The study provides the first near-surface geophysical characterisation of potentially active basement faults in Ekiti State. Keywords: active fault, ERT, Ado-Ekiti, fault trace, basement geology