EAST-AFRICAN EARTHQUAKES BELOW 20-KM DEPTH AND THEIR IMPLICATIONS FORCRUSTAL STRUCTURE

We report source parameters for eight earthquakes in East Africa obtai ned using a number of techniques, including (1) inversion of long-peri od P and SH waves for moment tensors and source-time functions, (2) fo rward modelling of first-motion polarities and P and pP amplitudes on short-period seismograms, and (3) determination of pP-P and sP-P diffe rential traveltimes from short-period records. The foci of these earth quakes lie between depths of 24 and 34 km in Archean and Proterozoic l ithosphere, and all but one fault-plane solution indicates normal faul ting (primarily E-W extension), consistent with the regional stress re gime in East Africa. Because many of these earthquakes occurred in are as where the crust may have been thinned by rifting, it is difficult t o ascertain whether or not their foci lie within the lower crust or up per mantle. Some of them, however, occurred away from rift structures in Proterozoic crust that is possibly 35-40 km thick or thicker, and t hus they probably nucleated within the lower crust. Strength profile c alculations suggest that in order to account for seismogenic (i.e. bri ttle) behaviour at sufficient depths to explain lower crustal earthqua kes in East Africa, the lower crust must not only be composed of mafic lithologies, as suggested by previous investigators, but also that si gnificantly more heat (similar to 100 per cent) must come from the upp er crust than predicted by the crustal heat source distribution obtain ed from a 1-D interpretation of the linear relationship between heat f low and heat production observed in Proterozoic terrains within easter n and southern Africa. Precambrian mafic dike swarms throughout East A frica provide evidence for magmatic events which could have delivered large amounts of mafic material to the lower crust over a very broad a rea, thus explaining why the lower crust in East Africa might be mafic away from the volcanogenic rift valleys.