Crustal shear wave velocity structure of Tanzania from ambient seismic noise tomography and the thicknesses of Karoo and younger basins in Southeastern Tanzania

Open Access
Author:
Boyle, Katie Rose
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
August 17, 2012
Committee Members:
  • Andrew Arnold Nyblade, Thesis Advisor
  • Charles James Ammon, Thesis Advisor
  • Peter Christopher Lafemina, Thesis Advisor
Keywords:
  • ambient seismic noise
  • tomography
  • East Africa
  • shear wave
  • crustal structure
  • Karoo
  • Southeastern Tanzania
  • sedimentary
  • basin
Abstract:
The thickness of the Karoo (Permian-Jurassic) sedimentary basins and the Jurassic- Neogene coastal sedimentary basins in southeastern Tanzania are investigated in this study. The Karoo basins consist mainly of siltstones and sandstones, while the coastal sedimentary basins consist mainly of clays and marls. Cross-correlation functions of ambient seismic noise recorded from 2007-2011 were calculated between 77 seismic stations located in Tanzania, Zambia, Kenya, and Uganda. Rayleigh wave group velocity dispersion curves were obtained from the cross-correlation functions, as well as from over 200 earthquakes recorded between 1990 and 2009 at 23 stations. The group velocities at periods of 8 to 30 seconds were inverted to create group velocity maps for the study area. Using these maps, group velocity dispersion curves were obtained for equally spaced grid nodes across the region and then inverted with phase velocity dispersion curves from Adams et al. (2012) for crustal shear wave velocity structure. Experimental results from other studies show that velocities in siltstones and sandstones (sediments comprising much of the Karoo basin) reach velocities as high as 2.8-3.0 km/s and shear wave velocities of clays and marls (sediments comprising much of the coastal basins) reach velocities as high as 2.5-2.7 km/s. Using upper limits of 3.0 km/s and 2.7 km/s for the average shear wave velocities for the Karoo and coastal sediments, the thicknesses of the basins were estimated. Results indicate basin thicknesses of 5-7 km and 3-5 km for the Karoo and coastal basins, respectively. These thicknesses suggest that 1) the Karoo basins lie deep within the oil window and into the gas window and could have a high potential for hydrocarbon production, and 2) the coastal basins lie within the upper oil window and could have potentially produced hydrocarbons if the sediments reached high enough temperatures.