# Preliminary Seismic Hazard Analyses for the Ugandan Region
## 4 Results  
### Seismic Source Characterization  
Compiling earthquake data from the ISC Reviewed Bulletin resulted in 1092 events spanning 1912 to 2020. Removing foreshocks and aftershocks netted a final catalog of 929 earthquakes in the Ugandan region. The epicenters of these earthquakes are mapped in Figure 3. The epicenters suggest the region can be split into four zones of visually similar seismicity. These zones are numbered 1 to 4, with seismic zone 1 being the northwestern branch of the East African Rift System, with a highly concentrated epicenter distribution and evidence of faults like Buganda-Toro fault, Bundibugyo fault, Butiaba fault, North Toro Bunyoro fault, Bwambara fault, Ibimbo fault, Nyamwamba fault, Kisomoro fault, Tonya fault, and Bunia Border fault in the western rift segment. The presence of concentrated seismic activities around the Lake Victoria basin guided the zoning for seismic segment zone 2. The remaining areas were designated as seismic zone 3, with which covers most of Uganda, and seismic zone 4, which covers the Democratic Republic of Congo in the west. Source types were only considered because outside of the rift zone, there appears insufficient data to derive necessary parameters for the hazard integral. Within the rift zone, there are multiple faults of varying sizes in close proximity, which makes modeling the rift zone as a zone more appropriate.  
Figure 2: Logic tree used in this study.  
Using these demarcations, the seismicity of each seismic zone is examined by estimating the recurrence rate for each zone. By tallying the number of earthquakes in each zone and plotting the number of earthquakes greater than a certain magnitude results in parameterization of the Gutenberg-Richter recurrence rate model. An example is shown for seismic zone 1 in Figure 4. The regressed \(a\) and \(b\) parameters are also indicated in the figure.  
Table 3 presents the a and b parameters for the Gutenberg-Richter recurrence rate model for each seismic zone. The earthquake catalog suggests there are approximately 1.51, 0.36, 0.71, and 0.11 earthquakes per year on average in seismic zones 1, 2, 3, and 4, respectively. Interestingly, the \(b\) values suggest seismic zone 1 and 4 have relatively more larger magnitude earthquakes than the other zones.  
Figure 4: Example Gutenberg–Richter recurrence rate model for seismic zone 1.  
Figure 3: Earthquake epicenters in the Ugandan region for (1912–2020).  
An important part of source characterization is defining the maximum magnitude that can be produced from each source. Previous studies have recommended increasing the maximum observed magnitude from a seismic source by 0.5 [28; 33]. The \(M_{max,observed}\) ranges from 5.4 to 7.0 in the entire region. Along the western branch of the rift segment in zone 1, a maximum observed magnitude of \(M_{W}=7.0\). Around the Victoria basin, a \(M_{max,observed}=5.4\) was also recognized and this could be a result of minimal seismic activities. Outside of the rift segment and Victoria basin, a \(M_{max,observed}=6\) and 5.5 for seismic zones 3 and 4, respectively. Table 4 lists the minimum and maximum magnitudes used in this study.