Contrasted seismogenic and rheological behaviours from shallow and deep earthquake sequences in the North Tanzanian Divergence, East Africa

We report preliminary results of a seismological experiment, SEISMO-TANZ’ 07, which consisted in the deployment of a local network (35 stations) in the East African Rift System (EARS), North Tanzania, during 6 months in 2007. We compare two earthquake sequences (Gelai and Manyara) occurring, respectively, in the southern end of the Kenya rift and in the North Tanzanian Divergence (NTD). Only distant of ∼150 km, their triggering mechanisms are different. None of the sequences depicts typical swarm or mainshock–aftershock patterns. They highlight the change in the magmatic/tectonic nature of the rift where the eastern branch of the EARS enters the Tanzanian craton. The similar shape and long-axis of the elongate sequences emphasize the preferred locus of active strain release along NE–SW discontinuities which probably root at depth into steep Proterozoic shear zones. At Gelai, the deformation is dominated by aseismic process involving slow slip on normal fault and dyke intrusion within the upper crust (Calais et al., 2008). The spatial and temporal earthquake distribution indicates a possible correlation between the Gelai crisis and the eruption of the nearby Oldoinyo Lengai volcano. At Manyara, the sequence is more uncommon, revealing a long-lasting seismic activity deeply rooted (∼20–35 km depth) possibly related to stress loading transmitted laterally. The yield strength envelope modelled from the depth frequency distribution of earthquakes in the NTD is consistent with the presence of a mafic lower crust and further supports the strength increase of the rifted crust from south Kenya to the NTD.