Browsing by Author "Mulibo G.D., and Nyblade A.A"

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    THE 1994–1995 MANYARA AND KWAMTORO EARTHQUAKE SWARMS: VARIATION INTHE DEPTH EXTENT OF SEISMICITY IN NORTHERNTANZANIA
    (GeoScienceWorld, 2009-12) Mulibo G.D., and Nyblade A.A
    Seismicity within two earthquake swarms (mb 1.0 to 4.0) in northern Tanzania recorded by the 1994-1995 Tanzania Broadband Seismic Experiment has been investigated through event relocation, modelling of regional depth phases and examination of focal mechanisms. In the Manyara swarm, seismicity is distributed over a region ~20 km wide and extends to >30 km depth. Hypocenters correlate well with the Manyara Rift, and focal mechanisms of many events show normal faulting with nodal planes having the same north-northeast orientation as the Manyara Rift border fault. This result indicates that the events are consistent with slip along the border fault and related faults beneath the Manyara Rift, and shows that the faults extend into the lower crust. Seismicity in the KwaMtoro swarm is distributed over a region ~10 km wide but extends only to ~12 km depth. There are no mapped faults above the swarm, but a strong correlation between the north to south orientation of the swarm, the north to south orientation of nodal planes in focal mechanisms, and north to south striking extensional structures nearby, suggest that events in this swarm could be caused by slip on a system of rift faults. However, a magma-driven origin for either swarm cannot be ruled out either.
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    Mantle transition zone thinning beneath eastern Africa: Evidence for a whole-mantle superplume structure
    (Wiley, 2013-08-22) Mulibo G.D., and Nyblade A.A
    [1] P to S conversions from the 410 and 660 km discontinuities observed in receiver function stacks reveal a mantle transition zone that is ~30–40 km thinner than the global average in a region ~200–400 km wide extending in a SW-NE direction from central Zambia, across Tanzania and into Kenya. The thinning of the transition zone indicates a ~190–300K thermal anomaly in the same location where seismic tomography models suggest that the lower mantle African superplume structure connects to thermally perturbed upper mantle beneath eastern Africa. This finding provides compelling evidence for the existence of a continuous thermal structure extending from the core-mantle boundary to the surface associated with the African superplume.
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    The Seismotectonics of southeastern Tanzania: Implication for the propagation of the Eastern Branch of the East African Rift
    (Wiley, 2016-04-02) Mulibo G.D., and Nyblade A.A
    Seismicity patterns and focal mechanisms in southeastern Tanzania, determined from data recorded on temporary and permanent AfricaArray seismic stations, have been used to investigate the propagation direction of the Eastern branch of the East African Rift System southward from the Northern Tanzania Divergence Zone (NTDZ). Within the NTDZ, the rift zone is defined by three segments, the Eyasi segment to the west, the Manyara segment in the middle, and the Pangani segment to the east. Results show that most of the seismicity (~ 75%) extends to the south of the Manyara segment along the eastern margin of the Tanzania Craton, and at ~ 6–7° S latitude trends to the SE along the northern boundary of the Ruvuma microplate, connecting with a N–S zone of seismicity offshore southern Tanzania and Mozambique. A lesser amount of seismicity (~ 25%) is found extending from the SE corner of the Tanzania Craton at ~ 6–7° S latitude southwards towards Lake Nyasa. This finding supports a model of rift propagation via the Manyara segment to the southeast of the Tanzania Craton along the northern boundary of the Ruvuma microplate. However, given the limited duration of the seismic recordings used in this study, the possibility of another zone of extension developing to the south towards Lake Nyasa (Malawi) cannot be ruled out. Focal mechanisms along the boundary between the Victoria and the Ruvuma microplates and offshore southeastern Tanzania show a combination of normal and strike slip faulting indicating mainly extension with some sinistral motion, consistent with the mapped geologic faults and a clockwise rotation of the Ruvuma microplate.