Earthquakes along the East African Rift System: A multiscale, system-wide perspective

On the basis of a comprehensive data set of precisely determined depths of 121 large to moderate-sized earthquakes along and near the entire East African Rift System ( EARS), there are three distinct patterns in focal depths which seem to correlate with progressive stages in the development of the largest active rift in the world. First, away from both ends of the western, younger branch of the EARS, very large (M-w >= 7) earthquakes occurred in the top 15 km of the crust where surficial expressions of rifting are yet to appear. Curiously, there are unusually deep aftershocks reaching down to 35 +/- 3 km. Second, under well-developed but amagmatic rift segments, focal depths show a bimodal distribution, with peaks centered near depths of about 15 +/- 5 km and 35 +/- 5 km. This pattern is present both under the main axis of the EARS, where rift zone have lengths approaching 1000 km, and beneath rift units 10 times shorter in length. Underside reflections off the Moho indicate that at least part of the second peak in seismicity is due to mantle earthquakes down to 44 +/- 4 km, attesting to high differential stress in the mantle lithosphere which is capable of accumulating seismogenic, elastic strain (the jelly sandwich rheology). Third, beneath magmatic segments of well-developed rifts, seismicity is largely confined to the upper 15 km of the crust as observed previously, akin to the pattern along mid-ocean ridges where plastic flow due to high temperature inhibits accumulation of shear stress deep in the lithosphere.