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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, Hstacking and waveform inversion, yielding Moho depth estimates of 34 ± 2 km (Hstacking) and 38 ± 2 km (waveform inversion), and a crustal Poisson’s ratio of 0.27 ± 0.01 (Hstacking). 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 sufficiently strong at midand lower crustal depths to support brittle deformation. SOUTH AFRICAN JOURNAL OF GEOLOGY, 2009, VOLUME 112 PAGE 251-260 doi:10.2113/gssajg.112.3-4.251 SOUTH AFRICAN JOURNAL OF GEOLOGY CRUSTAL VELOCITY STRUCTURE OF THE RUKWA RIFT, EAST AFRICAN RIFT SYSTEM 252 rheological model (Kirby, 1980; Brace and Kohlstedt, 1980, Jackson, 2002) for brittle failure to occur at depths of > 20 km in east Africa, where average heat flow is about 60 to 65 mWm (Nyblade, 1997). They concluded that the lower crust must have a mafic composition, otherwise the lower crust would not be sufficiently strong to support brittle failure. Further to that conclusion, Julia et al. (2005) examined details of crustal structure in Tanzania by jointly inverting receiver functions and Rayleigh wave phase and group velocities using data recorded by the Tanzania broadband seismic experiment stations (Nyblade et al., 1996). Julia et al. (2005) found shear wave velocities of >3.9 km/s in the bottom ~5 to 8 km of the crust beneath many stations away from the rift valleys, indicating the presence of mafic lithologies. Given this result, earthquakes in east Africa nucleating within the lowermost part of the crust outside of the rifts can be explained by brittle deformation within mafic rock, provided that temperatures in the lowermost crust have not been greatly elevated by the extensional tectonism. But this finding leaves open the question of why earthquakes beneath the rift valleys nucleate at depths between about 20 km and the bottom 5 to 8 km of the crust, where the crustal composition is less likely to be mafic. One possibility is that beneath some of the rifts a larger portion of the lower crust is mafic than beneath the unrifted terrains. And given that possibility, the second purpose of this paper is to obtain an estimate of the average seismic velocity structure of the Rukwa rift crust to determine whether or not velocities below about 20 km depth are consistent with the presence of mafic lithologies. To estimate the thickness and velocity structure of the Rukwa Rift crust, we first examine crustal structure at the southeastern end of the rift through a re-analysis of receiver functions for station PAND (Figure 1) using two methods, Hstacking (Zhu and Kanamori, 2000) and waveform inversion (Ammon et al., 1990). We then model regional waveforms from the Mw 5.9 August 18, 1994 Rukwa earthquake recorded on station PAND (Figure 1) to obtain a 1D velocity model for the rifted crust. Background The Rukwa Rift is located in the Proterozoic Ubendian mobile belt to the south of the Archean Tanzania Craton (Figure 1). The rift is surrounded by exposed basement Figure 1. Map of the Rukwa Rift showing the location of the 18 August 1994 Lake Rukwa earthquake (star) and seismic stations of the 199495 Tanzania broadband seismic experiment (triangles). The gray colored area in the right-upper inset shows location of map with respect to the main Precambrian terrains (Tanzania Craton, Ubendian Belt and Mozambique Belt), and political boundaries. The bold black lines and the gray dashed lines denote border faults and seismic reflection lines, respectively. S. KIM, A.A. NYBLADE AND C-E. BAAG SOUTH AFRICAN JOURNAL OF GEOLOGY 253 rock and is bounded to the northeast and southwest by two major border faults (Lupa and Ufipa, Figure 1). The upper crustal structure of the Rukwa Rift has been investigated in a number of studies (e.g. Peirce and Lipkov, 1988; Kilembe and Rosendahl, 1992; Morley et al., 1992; Mbebe, 1993; Wheeler and Karson, 1994). Interpretations of seismic reflection profiles show rift faults extending to a depth of at least 10 km (Morley et al., 1992; Mbebe, 1993) while seismicity studies of lower crustal earthquakes suggest that faulting may extend to lower crustal depths (Camelbeeck and Iranga, 1996; Zhao et al., 1997). The region has been affected tectonically several times since the Late Neoproterozoic, and the sedimentary record reflects periods of tectonically controlled sedimentation during the Karoo, Cretaceous, Paleogene, and Neogene-Quaternary (Dypvik et al. 1990; Roberts et al., 2004). The Karoo Supergroup consists predominantly of continental deposits (conglomerates, sandstones, shales, and coal) sitting on Precambrian basement. The Karoo sediments are overlain by the Cretaceous-Paleogene Red Sandstone group, and at the top of the stratigraphic sequence are unconsolidated Neogene-Quaternary lake deposits. The total thickness of the sedimentary layers is 5 to 10 km (Morley et al. 1992; Mbebe, 1993). Volcanism during the Neoproterozic, Cretaceous and Miocene occurred within the vicinity of the Rukwa Rift (Rasskazov et al. 2003; Ebinger, 1989), and periods of tectonic inversion and lateral shearing occurred during the Late Neoproterozoic and Permo-Triassic (Delvaux, 2001; Ebinger, 1989; Klerkx et al., 1998; Lenoir et al., 1994; Theunissen et al., 1996). Data The data used in this study come from the Tanzania broadband seismic experiment (Nyblade et al. 1996). In this experiment, twenty seismic stations were deployed for one year (1994-1995) across Tanzania, with station PAND located at the southeastern end of the rift (Figure 1). Other nearby stations included TUND, AMBA Table 1. List of teleseismic events used for computing receiver functions Date Time Latitude Longitude BAZ Magnitude Depth Fit (yyyymmdd) (hhmm) (deg) (deg) (Mb) (km) (%) 19940615 0923 -10.34 113.66 79.1 98.8 6.0 19.9 97.2 19940630 0923 36.33 71.13 57.4 36.0 6.5 226.6 93.2 19940725 2200 -56.36 -27.36 66.5 211.9 6.3 81.3 90.1 19940808 2108 24.72 95.2 69.0 59.3 6.0 121.7 91.5 19940915 2347 -57.8 -8.77 58.4 204.9 5.2 10 79.8 19940916 0620 22.53 118.71 89.3 67.2 6.5 13.1 98.7 19940928 1639 -5.79 110.35 76.4 93.8 5.9 637.5 96.6 19941025 0054 36.36 70.96 57.4 35.9 6.2 238.7 82.9 19941031 1148 3.02 96.19 63.9 82.2 5.7 29.4 83.7 19941114 1916 13.52 121.07 90.0 76.3 6.1 31.5 97.4 19941115 2018 -5.59 110.19 76.3 93.6 6.2 560.6 96.8 19950103 1612 -57.7 -65.88 87.3 212.1 6.2 13.9 78.