The Western Africa model (WAF) was developed internally by GEM. The model encompasses the whole Atlantic side of the Africa continent. An ad-hoc homogenised earthquake catalogue was developed based on globally available information, which was used as primary base for seismic occurrence analysis and the subsequent development of the source zonation model. The analysis was particularly challenging in the region, due to the severe incompleteness of calibration data, and the virtually nonexistent neotectonic information.
Information about the OQ model versions and input files can be found on the Results and Dissemination page.
The viewer below depicts the seismic sources and hazard results in terms of PGA for a return period of 475 years. Click on the menu in the upper right corner to select the layer.
Western Africa has very little tectonic activity, with low rates and magnitudes of seismicity. A few old faults within the African craton may be episodically reactivated, such as in the Cameroon Volcanic Line, but these earthquakes are somewhat rare and poorly understood.
GEM has created a new Mw-homogenised earthquake catalogue by assembling globally (ISC review bulletin, GCMT, ISG-GEM, GHEC) and locally available sources (Ghana catalogue, Amponsah et al., 2012). The GEM implementation of the Earthquake Catalogue for Central Africa (hereinafter GEM-CAEC) consists of 114 events with 4 ≤ Mw ≤ 6.5, covering a period from 1636 to 2013 (Figure 1).
Figure 1 - Seismic zones of the Western Africa model (in yellow) and the GEM earthquake catalogue for Central Africa (GEM-CAEC). The limit of the catalogue selection area is marked by the dashed line.
Seismic Source Characterisation
Area Source Zonation
The seismic source model of the WAF model consists of six area source zones (Figure 1). Seismicity in each area source is assumed to follow a double truncated Gutenberg-Richter magnitude occurrence relation (or magnitude- frequency distribution, MFD). Lower truncation is arbitrarily assigned to Mw 4.5. Due to the scarcity of calibration data, a unique Gutenberg-Richter b-value has been calculated from all events in the study region. Conversely, occurrence rates (a-values) have been calculated separately for each source zone by imposing the previously calibrated b-value. A different maximum magnitude (Mw-Max) estimate is derived independently for each source zone as the largest observed event plus an arbitrary - although quite conservative - increment of 0.5 magnitude units. Seismicity parameters are summarised in Table 1.
Table 1 - Seismicity parameters used in the WAF model.
To better represent the spatial variability of seismicity across the study area, the annual occurrence rates previously obtained for the homogeneous source zones were redistributed within each polygon using a procedure that accounts for the irregular spatial pattern of the observed events (Figure 2). The procedure shares some similarity with the popular smoothed seismicity approach (e.g. Frankel, 1995), but is more convenient in that a unique fit of the MFD is required for each zone, while the corresponding total earthquake occurrence is a-posteriori spatially reorganised as a function of the epicentral distance to all neighbouring events. Moreover, the combined use of zones gives the possibility to account for different modelling parameters (b-value, depth distribution, rupture mechanism) in separate regions.
Figure 2 - Spatial redistribution of the cumulative annual rates (M > 0) using a smoothing parameter (λ) of 100.
Ground Motion Characterisation
Table 2 shows the ground motion logic tree. The tectonic region type for WAF was assumed to be stable continental crust (Tectonic_Type_A).
Given the lack of calibration data and a local ground motion prediction model, we used the GMPEs (Atkinson and Boore, 2006; Pezeshk et al., 2011) selected by Poggi et al. (2017) for the stable continental regions of the Sub-Saharan Africa model.
Table 2 - GMPEs used in the WAF model.
Hazard curves were computed with the OQ engine for peak ground acceleration (PGA) and spectral acceleration (SA) at 0.2s, 0.5s, 1.0s, and 2s. The computation was performed on a grid of 124441 sites (spaced at approximately 10 km) with reference soil conditions corresponding to a shear wave velocity in the upper 30 meters (Vs30) of 760-800 m/s. Hazard curves are shown for Accra in Figure 3.
Figure 3 - Hazard curves calculated at different spectral periods for the city of Accra, capital of Ghana.
The hazard map for PGA corresponding to a 10% probability of exceedance in 50 years (475 year return period), can be seen using the interactive viewer. For a more comprehensive set of hazard and risk results, please see the GEM Visualization Tools.
Amponsah, P., Leydecker, G. & Muff, R., 2012. Earthquake catalogue of Ghana for the time period 1615–2003 with special reference to the tectono-structural evolution of south-east Ghana. Journal of African Earth Sciences, Vol. 75, pp. 1-13
Atkinson G, Boore D (2006) Earthquake ground-motion prediction equations for eastern North America. Bull Seismol Soc Am 96:2181–2205
Frankel, A. (1995). Mapping seismic hazard in the Central and Eastern United States. Seismological Research Letters 66:4, 8-21.
Pezeshk S, Zandieh A, Tavakoli B (2011) Hybrid empirical ground-motion prediction equations for eastern North America using NGA models and updated seismological parameters. Bull Seismol Soc Am 101:1859–1870
Poggi, V., Durrheim, R., Mavonga Tuluka, G., Weatherill, G., Gee, R., Pagani, M., Nyblade, A., Delvaux, D., 2017. Assessing Seismic Hazard of the East African Rift: a pilot study from GEM and AfricaArray. Bulletin of Earthquake Engineering. doi:10.1007/s10518-017-0152-4.