Site Dependence Earthquake Spectra Attenuation Modeling: Nigerian Case Study

Abstract

Recent seismic events recorded in South-western Nigeria indicate that the country may not be aseismic as had hitherto thought. Geologic and geodetic evidences suggest the existence of large fracture zones (Romanche and Charcot) beneath the area. Considering the existence of these fracture zones, and the paucity of seismicity information, the development (oil exploration and production) taking place in offshore Nigeria in the last two decades and the ambitious planning for large future projects urgently call for the implementation of a comprehensive earthquake ground motion modelling which is a useful tool in site-dependent seismic hazard assessment in low to moderate seismicity region. In this study, ground-attenuation modelling based on stochastic approach was applied to predict the expected peak ground velocity and acceleration and spectral amplifications in two geologic settings. The seismic ground motion has been modelled using the September 11, 2009 earthquake of magnitude 4.8 (Mw) as case study. Synthetic seismic waveforms from which parameters for engineering building design could be obtain have been derived. From the seismograms computed, the seismic hazard for south-western Nigeria, expressed in terms of peak ground acceleration and peak ground velocity have been estimated. The peak ground acceleration estimated for the study area ranges from 0.16 to 0.69 g, and the peak ground velocity from 18.0 to 58.3 m/sec. The high peak values of accelerations and amplifications delineated are possibly due to the presence of the low velocity layers. In general, a good correlation between the synthetic and field data was observed. These results attest to the efficacy of the modelling exercise, and assessment of the seismic risk that the region would likely be subjected to. Also, the earthquake engineering design parameters derived may be used to derive new civil engineering building codes for the affected area.