# Preliminary Seismic Hazard Analyses for the Ugandan Region
## 4 Results
### Calculating the Hazard  
To demonstrate the results of the hazard calculation, two sites as shown in Figure 1, Site 1 and Site 2 will be used as an example. These sites are of interest as they have been noted in planned nuclear power infrastructure projects. The results from PSHA estimations of the hazard for PGA at Site 1 is shown in Figure 5. The figure breaks it down into the results from the Toro GMPE [29] in Figure 5a and the Idriss GMPE [30] in Figure 5b. Figure 5 suggests the hazard contribution from using the Toro GMPE [29] is generally lower than the hazard calculations using the Idriss [30] GMPE.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Seismic Zone** & \(M_{min}\) & \(M_{max,observed}\) & \(M_{max}\) \\ \hline
1 & 4.0 & 7.0 & 7.5 \\
2 & 4.0 & 5.4 & 5.9 \\
3 & 4.0 & 6.0 & 6.5 \\
4 & 4.0 & 5.5 & 6.0 \\ \hline \hline \end{tabular}
\end{table}
Table 4: \(M_{min}\) and \(M_{max}\) for each seismic zone.  
Figure 5: PSHA resultant hazard curves for Site 1 considering (**a**) Toro [29] and (**b**) Idriss [30] GMPEs.  
As PGA is the intensity measure of interest in this study, the total hazard for PGA at Sites 1 and 2 are shown in Figure 6. This figure suggests that Site 2 can experience a higher PGA relative to Site 1. For example, Site 1 is expected to have 0.007 earthquakes per year on average with a PGA exceeding 0.1 g, while Site 2 is expected to have 0.01 earthquakes per year on average with a PGA exceeding 0.1 g. Alternatively, an earthquake with a PGA greater than 0.1 g is expected to have a return period of 136 years at Site 1 but a return period of 76 years at Site 2. So even though Site 2 is farther away from the western branch of the East African Rift Valley as represented by seismic zone 1, the analyses imply Site 2 is slightly more susceptible to stronger earthquakes than Site 1.  
To help quantify this behavior, the seismic hazards from 575 sites are compiled to construct seismic hazard maps. Seismic hazard maps are typically designed to show the intensity measure at a specific hazard level. The hazard levels are described as probabilities of exceedance. In this study, these are 10% exceedance in 5, 10, 20, 50, 100, and 500 years, which correspond to earthquakes with return periods of approximately 50, 100, 200, 500, 1000, and 5000 years. Figure 7 shows the seismic hazard contour maps for 10% exceedance in (a) 5, (b) 10, (c) 20, (d) 50, (e) 100, and (f) 500 years. The maps show that the horizontal PGA values of 10% exceedance for Uganda fall in the range of 0.02 to 0.28 g, 0.02 to 0.36 g, 0.02 to 0.42 g, 0.04 to 0.52 g, 0.05 to 0.65 g, and 0.05 to 0.95 g for return periods of 50, 100, 200, 500, 1000, and 5000 years, respectively. The figure also indicates a progression in the PGA contours as the expected return period lengthens. These suggest the areas related to seismic zone 1 exhibits the highest levels of shaking with a concentration east of the Rwenzori mountains and Lake Albertine. These also suggest the lowest levels of expected shaking occur in the northwestern area outside of Uganda in seismic zone 4 and the northeastern area of Uganda in seismic zone 3.  
Figure 6: PSHA hazard results for PGA at Sites 1 and 2.