An Evaluation of the 11^th September, 2009 Earthquake and Its Implication for Understanding the Seismotectonics of South Western Nigeria

Abstract

The evaluation of an intra-plate earthquake that occurred on Friday, 11 th September, 2009 and felt in parts of Abeokuta, Ago-Iwoye, Ajambata, Ajegunle, Imeko, Ijebu-Ode, Ilaro and Ibadan, all in south western Nigeria is presented. This event has been the largest inland earthquake recorded since the inception of digital seismological recording in Nigeria in 2008 was incepted. The event was recorded by three seismological stations operated by Centre for Geodesy and Geodynamics (CGG), Toro. Data obtained from the CGG stations and others distributed around the world were analysed to determine precise earthquake locations and focal mechanism and to assess the regional tectonic stress. The data recorded in MiniSEED format at a sampling rate of 40 samples per second (sps) were analyzed using the SEISAN earthquake analysis software. The resultshowed an epicentral location situated about 108 km west of Lagos, a focal depth of 10.0 km and an origin time of 03:10:21.60 GMT. The local and moment magnitudes were 4.5 and 4.2 respectively. The P-wave to S-wave velocity ratio was 1.72. The fault plane solutions obtained for the rupture process indicated that a normal dip-slip fault with median solution of strike 325 ° , dip 40 ° and rake -90 ° was the probable trigger mechanism for this earthquake. It suggested that the event was a reactivation of a buried high-angle fault in the Precambrian basement represented by the contemporary northeast-southwest trending regional horizontal compressive stress. Generally, the seismotectonics of the region was linked to the fracture zones in the Atlantic Ocean.