ALBERT

Description: SUMMARY We use teleseismic waveform inversion, along with depth phase analysis, to constrain the centroid depths and source parameters of large African earthquakes. The majority of seismic activity is concentrated along the East African Rift System, with additional active regions along stretches of the continental margins in north and east Africa, and in the Congo Basin. We examine variations in the seismogenic thickness across Africa, based on a total of 227 well-determined earthquake depths, 112 of which are new to this study. Seismogenic thickness varies in correspondence with lithospheric thickness, as determined from surface wave tomography, with regions of thick lithosphere being associated with seismogenic thicknesses of up to 40 km. In regions of thin lithosphere, the seismogenic thickness is typically limited to ≤20 km. Larger seismogenic thicknesses also correlate with regions that have dominant tectonothermal ages of ≥1500 Ma, where the East African Rift passes around the Archean cratons of Africa, through the older Proterozoic mobile belts. These correlations are likely to be related to the production, affected by method and age of basement formation, and preservation, affected by lithospheric thickness, of a strong, anhydrous lower crust. The Congo Basin contains the only compressional earthquakes in the continental interior. Simple modelling of the forces induced by convective support of the African plate, based on long-wavelength free-air gravity anomalies, indicates that epeirogenic effects are sufficient to account for the localization and occurrence of both extensional and compressional deformation in Africa. Seismicity along the margins of Africa reflects a mixture between oceanic and continental seismogenic characteristics, with earthquakes in places extending to 40 km depth.