Seismicity of the East African Rift System for the period 2013 to 2016

Space-time distribution of earthquakes in the East African Rift System (EARS) has been investigated for the years 2013-2016. The spatial earthquakes distribution in the time window considered in this study has revealed that the Main Ethiopian Rift (MER) is structurally linked to the eastern branch of the EARS. A relatively high level of seismicity has been obtained in this study compared to the ISC (International Seismological Centre) catalog for the same time period which implies the region is seismically more active than we thought. In addition to the rift floor, the rift margins and the surrounding plateaus are found to be seismically active. The western and eastern branches of the EARS are found to be more active than the other rift segments for the time period considered. The frequency-magnitude distribution (FMD) of events during this study period provided an average b- and a -values of -1.01 and -6.5, respectively. The b-value result is implying relatively high stress with tectonic origin earthquakes occurrence dominating the region. On the other hand, threshold magnitude completeness of Mc 3.0 is obtained which is the minimum value that has never been observed previously for such large area. The seismic energy map of the region has been systematically investigated and peaks energy-release is observed at Afar depression, eastern and western branches. The energy mapping shows that the MER is structurally connected to the eastern branch of the EARS which is consistent with seismicity distribution. The overall depth of occurrence of events in the eastern and the western branches of the EARS are relatively deeper than the one found for Afar and MER. This implies that the seismogenic thickness increases in EARS when we go from north to south which is in agreement with rift age, magmatism and influence of the African Super plume on the crust and upper mantle deformation in EARS.