Seismic hazard assessment of the Kivu rift segment based on a new seismotectonic zonation model (western branch, East African Rift system)

In the frame of the Belgian GeoRisCA multi-risk assessment project focusing on the Kivu and northern
Tanganyika rift region in Central Africa, a new probabilistic seismic hazard assessment has been performed
for the Kivu rift segment in the central part of the western branch of the East African rift system.
As the geological and tectonic setting of this region is incompletely known, especially the part lying in
the Democratic Republic of the Congo, we compiled homogeneous cross-border tectonic and neotectonic
maps.
The seismic risk assessment is based on a new earthquake catalogue based on the ISC reviewed
earthquake catalogue and supplemented by other local catalogues and new macroseismic epicenter data
spanning 126 years, with 1068 events. The magnitudes have been homogenized to Mw and aftershocks
removed. The final catalogue used for the seismic hazard assessment spans 60 years, from 1955 to 2015,
with 359 events and a magnitude of completeness of 4.4. The seismotectonic zonation into 7 seismic
source areas was done on the basis of the regional geological structure, neotectonic fault systems, basin
architecture and distribution of thermal springs and earthquake epicenters.
The Gutenberg-Richter seismic hazard parameters were determined by the least square linear fit and
the maximum likelihood method. Seismic hazard maps have been computed using existing attenuation
laws with the Crisis 2012 software.We obtained higher PGA values (475 years return period) for the Kivu
rift region than the previous estimates. They also vary laterally in function of the tectonic setting, with
the lowest value in the volcanically active Virunga e Rutshuru zone, highest in the currently nonvolcanic
parts of Lake Kivu, Rusizi valley and North Tanganyika rift zone, and intermediate in the regions
flanking the axial rift zone.