Abstract
The evaluation of an intra-plate earthquake that occurred on Friday, 11th 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.
Highlights
The Nigerian landmass is not expected to be affected by naturally occurring earthquakes because it is located far from the major earthquake zones of the world, as the nearest active plate boundary lies far away at the Mid-Atlantic Ridge and considered a stable continental area
The 1984 event was recorded by five stations and the epicentre was located somewhere near Ijebu-Ode [4] whereas the 1990 event which was recorded by only one station in Nigeria had its epicentre assumed to be near Ijebu-Ode based on previous occurrences and the areas within which the vibrations were felt [1] [2]
The normal dip-slip fault could possibly be an after effect of the strike slip fault. This solution is similar to what was obtained for earthquakes that occurred in southern Ghana where majority of the epicentres were concentrated around the intersection of the coastal boundary faults and the Akwapim faults [24]-[26]
Summary
The Nigerian landmass is not expected to be affected by naturally occurring earthquakes because it is located far from the major earthquake zones of the world, as the nearest active plate boundary lies far away at the Mid-Atlantic Ridge and considered a stable continental area. The National Earthquake Information Centre (NEIC) of the United States Geological Survey (USGS) located the event at 6.224 ̊N and 5.147 ̊E, calculated a body wave magnitude (MB) of 4.5 and a focal depth of 10 km. The International Seismological Centre (ISC), United Kingdom located the same event at 6.290 ̊N and 5.070 ̊E, determined the body wave (MB) and surface wave (MS) magnitudes of 4.4 and 3.9 respectively as well as focal depth of 10 km. The 1984 event was recorded by five stations (one in Nigeria and four in Cote d’Ivoire) and the epicentre was located somewhere near Ijebu-Ode [4] whereas the 1990 event which was recorded by only one station in Nigeria had its epicentre assumed to be near Ijebu-Ode based on previous occurrences and the areas within which the vibrations were felt [1] [2]
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