Crustal velocity structure of the Rukwa Rift in the western branch of the east african Rift system

We investigate the thickness and seismic velocity structure of the Rukwa Rift crust by modeling seismograms from the 1994 Mw 5.9 Rukwa earthquake and teleseismic receiver functions recorded on a broadband seismic station at the southeastern end of the rift. Two methods have been used to model receiver functions, H-κ stacking and waveform inversion, yielding Moho depth estimates of 34 ± 2 km (H-κ stacking) and 38 ± 2 km (waveform inversion), and a crustal Poisson's ratio of 0.27 ± 0.01 (H-± stacking). A 1D average velocity model for the rifted crust has been obtained by trial-and-error modeling of the Rukwa earthquake seismograms. The best fitting velocity model obtained has a 4.5 km thick near-surface low-velocity section over a 33 km thick middle-to-lower crustal section. The velocities and thickness of the near-surface layers are consistent with the known thickness and composition of the sedimentary basin fill. The middle-to-lower crustal section is characterised by a linear velocity gradient with shear wave velocities of 3.9 km/s or higher at depths > 20 km. Crustal Poisson's ratio in the model is 0.27. The Moho depth in the model (37.5 km) is similar to the Moho depth obtained from analysing receiver functions, suggesting that there has been little, if any, crustal thinning beneath the rift compared to the southeastern edge of the rift. The Poisson's ratio of 0.27, in combination with the high velocities in the middle and lower crust, indicate that a significant component of the rifted crust has a mafic composition. This finding is important for understanding the occurrence of lower crustal earthquakes in east Africa, because it suggests that at least some of the crust in east Africa could be sufficiendy strong at mid- and lower crustal depths to support brittle deformation.