The July 28, 1984 southwestern Nigeria earthquake and its implications for the understanding of the tectonic structure of Nigeria

Abstract

On 28th July, 1984 at about 12 hours 13 m UT an earthquake with an epicentral intensity of about VI occurred in the southwestern part of Nigeria, an area that has always been believed to be seismically inactive. This event was recorded at the LAMTO seismic observatory in Ivory Coast and its epicentre was located at around Ijebu-Ode. In Nigeria, the event was recorded only by a temporary monitoring station at Ahmadu Bello University (ABU) in Zaria, a distance of about 640 km from the epicentre. Two subsequent tremors that were reported in Ijebu-Ode in early August, 1984 were not recorded at Ahmadu Bello University. Geological, geophysical and geodynamic studies indicate the possibility of the existence of large fracture zones trending generally in the NE-SW direction across the country. The recent earthquakes in southwestern Nigeria could therefore be evidence to support the existence of the Pelusium megashear system or similar fracture zones that penetrate deep into the continental crust of West Africa. There is therefore a need for more adequate coverage of the area by seismic stations to permit a precise location of future events and detailed data analysis which would help in identifying area where large scale crustal adjustments might be taking place.