[1] Crustal structure in Kenya and Ethiopia has been investigated using receiver function analysis of broadband seismic data to determine the extent to which the Cenozoic rifting and magmatism has modified the thickness and composition of the Proterozoic crust in which the East African rift system developed. Data for this study These results indicate that the crust beneath the Ethiopian Plateau has not been modified significantly by the Cenozoic rifting and magmatism, even though up to a few kilometers of flood basalts have been added, and that the crust beneath the rifted regions in Ethiopia has been thinned in many places and extensively modified by the addition of mafic rock. The latter finding is consistent with models for rift evolution, suggesting that magmatic segments with the Main Ethiopian Rift, characterized by dike intrusion and Quaternary volcanism, act now as the locus of extension rather than the rift border faults.

Abstract. The assertion of cratonic stability put forward in the model for deep continental structure can be tested by examining upper mantle structure beneath the Tanzania Craton, which lies within a tectonically active region in east Africa. Tomographic inversions of about 1200 teleseismic P and $ travel times indicate that high-velocity lithosphere beneath the Tanzania Craton extends to a depth of at least 200 km and possibly to 300 or 350 km. Based on the thickness of mantle lithosphere beneath Archean cratons elsewhere, it appears that the mantle lithosphere of the Tanzania Craton has not been extensively disrupted by the Cenozoic tectonism in east Africa, thus corroborating the assertion of cratonic stability in the model for deep continental structure. The presence of thick, high-velocity structure beneath the Tanzania Craton implies relatively low temperatures within the cratonic mantle lithosphere, consistent with relatively low surface heat flow. The thick cratonic keel is surrounded by low seismic velocity regions beneath the east African rifts that extend to depths below 400 km. Our models show a shear velocity contrast between the cratonic lithosphere and the uppermost mantle beneath the eastern branch of the rift system of about 5% to 6%, but from resolution experiments we infer that this contrast could be underestimated by as much as a factor of 1.5. We attribute about half of this velocity contrast to the depleted composition of the cratonic keel and the other half to thermal alteration of upper mantle beneath the rifts. Low-density structures that may be required to provide buoyant support for the elevation of the Tanzania Craton must reside at depths greater than about 300-350 km.

S U M M A R YWe report source parameters for eight earthquakes in East Africa obtained using a number of techniques, including (1) inversion of long-pcriod P and SH waves for momcnt tcnsors and source-time functions, (2) forward modelling of first-motion polarities and P and p P amplitudes on short-period seismograms, and (3) determination of p P -P and sP-P differential traveltimes from short-period records. The foci of these earthquakes lie between depths of 24 and 34km in Archean and Proterozoic lithosphere, and all but one fault-plane solution indicates normal faulting (primarily E-W extension), consistent with the regional stress regime in East Africa. Because many of these earthquakes occurred in areas where the crust may have been thinned by rifting, it is difficult to ascertain whether or not their foci lie within the lower crust or upper mantle. Some of them, however, occurred away from rift structures in Proterozoic crust that is possibly 35-40 km thick or thicker. and thus they probably nucleated within the lower crust. Strength profile calculations suggest that in order to account for seismogenic (i.e. brittle) behaviour at sufficient depths to explain lower crustal earthquakes in East Africa, the lower crust must not only be composed of mafic lithologies, as suggested by previous investigators, but also that significantly more heat (-100 per cent) must come from the upper crust than predicted by the crustal heat source distribution obtained from a 1-D interpretation of the linear relationship between heat flow and heat production observed in Proterozoic terrains within eastern and southern Africa. Precambrian mafic dike swarms throughout East Africa provide evidence for magmatic events which could have dclivered large amounts of mafic material to the lower crust over a very broad area, thus explaining why the lower crust in East Africa might be mafic away from the volcanogenic rift valleys.

Abstract.We apply a three-dimensional stacking method to receiver functions from the Tanzania

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