Crustal Structure Beneath the Ethiopian Plateau, Afar Depression, and the Main Ethiopian Rift from Receiver Functions

Between March 2000 and February 2002 a network of 25 broadband seismic stations was operated in Ethiopia to investigate crust and upper mantle structure using data from teleseismic earthquakes. Stations were evenly distributed between the regions of rifted crust and highlands adjacent to the rifts. We have used two methods to analyze receiver functions for crustal structure, the H-κ (Moho depth(H) and Vp/Vs ratio(κ )) method, and direct stacks of the waveforms. The former technique is based on the idea that a stack of the weighted sum of receiver function values at the arrival times for the Ps converted phase from the Moho and the first two multiple converted phases will attain their maximum when there is coherent stacking. The two approaches yield consistent results for most of the stations. Moho depths vary from 24 to 30 km within the Main Ethiopian Rift and the Afar Depression, and the crust is characterized by high Poisson's ratios of 0.30-0.35. Beneath the western and eastern plateaus on either side of the Main Ethiopian Rift, crustal thickness ranges from 32 to 46 km, and Poisson's ratio varies from 0.23 to 0.28, typical for Precambrian crust. Thus, these results suggest that Precambrian crustal structure beneath the Ethiopian Plateau has not been significantly modified by the Cenozoic rifting and volcanism, whereas the Precambrian crust beneath the rifted regions has been dramatically thinned in some places and compositionally altered by the addition of mafic rocks.