Inverstigations of Crustal Structure Structure and Seismo-Tectonics in the Western Branch of the East African Rift System

Sammanfattning: The most seismically active part of the East African rift system (EARS) is the western branch. Understanding the crustal structure and its evolution is important in order to obtain accurate locations and mechanisms of earthquakes, hence to deduce the nature of the neotectonics. This thesis uses information from earthquakes to investigate the crustal structure and active tectonics in the western branch of the EARS, centred on the Rukwa rift. An average 1-D crustal structure characterised by longitudinal and shear wave velocities, plus the quality factor, QLg, has been determined, using arrival times of body waves and maximum displacement amplitudes of Lg waves. This structure is used to determine the body-wave magnitude scale (mbLg) for the region and to establish the earthquake detection threshold for stations of the Mbeya seismic network located within the Rukwa rift. This thesis also develops a new approach for modelling full waveform, high frequency siesmograms to obtain fault plane solutions for micro-earthquakes in local inhomogeneous media. As it is possible to model low frequency data using a homogeneous medium, modelling high frequency seismic waves in inhomogeneous media requires characterisation of the media along different propagation paths between the source and stations. After checking that this approach produces reliable fault plane solutions for three micro-earthquakes, it was applied to micro-earthquakes along the Rukwa rift. Together with solutions for large earthquakes previously determined by other investigators, fault plane solutions for micro-earthquakes were used to constrain the kinematics of the Rukwa rift. It has been found that the T- (tension) axis is oriented NE along the Ufipa plateau that forms the western boundary to the Rukwa rift, and NW in the Songwe basin within the rift. This difference is attributed to differences in gravitational forces induced by differences in topography: the Ufipa plateau is about 1 km higher than the Songwe basin, the later being at the same level as the East African plateau.