THE 11TH SEPTEMBER 2009 ML 4.5 EARTHQUAKE AND ITS IMPLICATIONS FOR REGIONAL TECTONICS IN SOUTHERN NIGERIA

An intra-plate earthquake with local M_L 4.5 and Mw 4.3 moment magnitudes of occurred on 11th September 2009 in Southern Nigeria. This event is the largest inland earthquake to occur since the inception of the modern seismic observation system recording in Nigeria in 2006. Both the recorded local and the teleseismic events obtained from 20 stations around the world were subjected to detailed analysis in other to determine precise earthquake locations, focal mechanism solutions, and the current status of regional tectonic stress that it is associated with, and the crustal and mantle structure implications. An epicentral location of the earthquake was deduced to be at latitude 6.611° and longitude 2.433°; a focal depth of 10.0 km and an origin time of 3:10.21.60 GMT. A normal fault with median solution of strike 325°, dip 40° and rake -90° was possibly the trigger mechanism for the earthquake. Its source mechanism, suggest that the event possibly represents reactivation of a buried high-angle fault in the Precambrian basement by the contemporary -northeast –southwest trending regional horizontal compressive stress.

From the interpreted teleseismic events recorded by other networks located at different azimuths. The group velocities of the Rayleigh and Love waves with surface wave magnitude ≥ 6.0 in the period range of 20 – 100 s were inverted for plane-layered shear-velocity structure. From the inversion results, it is observed that, the shear-wave velocity of the crust beneath the investigated area increases gradually from about 3.23 to 4.11 km/s. The Moho was found at depth 32.5 km. The upper mantle shear-velocity increases from about 4.26 to 4.46 km/s between depth 32.5 and 87.5 km and a low velocity layer between depth 87.5 and 275 km. It is concluded that a site dependent seismic waves dispersion associated with the local geology is the region is implied.