TITLE:
An Evaluation of the 11th September, 2009 Earthquake and Its Implication for Understanding the Seismotectonics of South Western Nigeria
AUTHORS:
Ofonime U. Akpan, Monday A. Isogun, Tahir A. Yakubu, Abraham A. Adepelumi, Chiedu S. Okereke, Adetola S. Oniku, Michael I. Oden
KEYWORDS:
Intra-Plate Earthquake, Normal Fault, Epicentre, South Western Nigeria
JOURNAL NAME:
Open Journal of Geology,
Vol.4 No.10,
October
29,
2014
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.