Preliminary geological evidence for multiple tremors in Kwoi, Central Nigeria

Abstract

Preliminary field assessment of the geological evidence for the September 2016 multiple tremors was felt as far as 10 km in Nok and Chori villages in the north and northwest of Kwoi respectively. This involved systematic studies of the crack pattern and intensity on buildings, rocks and hanging walls of slopes around the area with the highest shock. The structural mapping was complemented by composite images of SPOT 5, supplementary subsurface investigations utilized aeromagnetic data, seismic data, and Digital Terrain Model (DTM) for the area. Results show that the major evidence that proved a possible link to the area of highest impact is the tremor triggered displacement of a 4 by 3 m diameter rock boulder situated about a kilometre from the Kwoi town, some 3 km from the nearest epicentre that fell through a cumulative distance of 25 m, splitting the fresh granite boulder into two and creating a high impact scar on its path. Seismic vibrations that shook the residents of Kwoi and environs (with epicentres located along a NE-SW linear traverse and remotely coinciding with buried fractures) appear to have been generated and propagated laterally from the rocks along a northwest-southeast profile. Despite the non-homogenous strengths of the impacted buildings, the near-consistency of the E-W striking walls being the most fractured gives a remote connection with the major 345° to 015° fracture patterns on the granitic plutons that shields Kwoi town to the north. Additionally, the intensity of the fractures and collapse in the buildings increased south-westwards from the perceived area of highest impact. A possible tectonic origin related to stress build up in the rocks of the area for the tremors can be insinuated thereby foreclosing the theory of a non-tectonic origin being considered in some quarters including the recent Mpape Abuja tremor of 2018. All these epicentres plot along an extrapolated trend that coincides with the Chain Fracture Zone of the North Atlantic.