Neotectonics of Brazzaville and Kinshasa: linking Congo Basin seismicity and in situ stress in the Inkisi Group

Abstract. The Congo Basin has been affected by several earthquakes for which the in-situ stress has not yet been reported. This study aims to determine the in-situ stress related to earthquakes in the Congo Basin, particularly those located in the north portion of the Republic of Congo (RC) and in the northwest portion of the Democratic Republic of Congo (DRC). The combined analysis of seismic history of the Congo Basin and of in-situ paleo-stress in the Inkisi Group allowed us to distinguish onshore earthquakes that are linked with preexisting zones of fractures on the continent and offshore earthquakes that are directly linked with transform faults. The Inkisi Group has been affected by two phases of strike-slip tectonics. The first phase, with a direction of compression N142°, is a result of the Gondwanide orogenesis in the Paleozoic. The second phase, with a compression direction of N078°, is related to the present-day stress of earthquakes in the Congo Basin. This phase is still active and is likely attributable to ridge push from the opening of the Atlantic Ocean. It is therefore appropriate that infrastructure construction in Brazzaville and Kinshasa considers seismic risk in the Inkisi bedrock of this area. As an example, we note that several masonry fences along the Congo river have developed fractures.