Research Programmes - Current Projects
AfricaArray’s current projects number more than a dozen, span many spatial and temporal scales, and are focused on the scientific needs of AA’s partners. National science organizations, government agencies, and private enterprises with interest or investment in Africa’s natural resource sector are among AA’s partners sponsoring those projects. Detailed information on several current projects is provided below.
Imaging the African Superplume
Principle Investigators: Andrew Nyblade, Penn State; Steve Grand, University
of Texas, Austin
Collaborators: Paul Dirks, Ray Durrheim, University of the Witwatersrand; Gerhard Graham, Council for Geoscience, South Africa
Sponsor: U.S. National Science Foundation (EAR-Geophysics Division)
The African superplume is both one of the most prominent and one of the most enigmatic features of the Earth’s mantle. Covering much of the southern African subcontinent, it is characterized by seismic wave velocities that are lower than other structures in the Earth’s lower mantle. The superplume also lies beneath an area with an anomalously high topography, suggesting a geodynamic relationship between the superplume and the formation of plateaus and rift valleys in eastern and southern Africa.
A better understanding of the nature and origin of the African superplume would significantly advance the field of tectonics and answer fundamental questions about how the mantle works. Using broadband seismic data, geoscientists could improve images of the African superplume and investigate the geodynamic relationship between it and mantle convection.
This project involves upgrading and expanding AfricaArray’s permanent seismic network through the purchase and installation of new equipment, both broadband seismometers and data loggers. This will add almost two dozen new stations across southern Africa, bringing the total number of stations in the backbone network to 50. The data and improved imaging provided by this expanded network will give a more complete image of the African mantle and hence, advance understanding of mantle dynamics.
Researchers’ activities include tomographic imaging of the upper and lower mantle using body wave travel times; modeling waveforms of teleseismic body wave phases that sample the superplume; jointly inverting receiver functions and surface wave dispersion measurements for crust and uppermost mantle structure; and stacking and migrating receiver functions to image topography on the 410 and 660 km discontinuities.
Developing and Exploiting a Unique Seismic Dataset from South African Gold Mines for Source Characterization and Wave Propagation
Principle Investigators: Andrew Nyblade, Jordi Julia, Penn State; Rengin Gok,
William R. Walter, Lawrence Livermore National Laboratory; Lindsay Linzer, Ray Durrhaim, Council
for Scientific and Industrial Research
Support: U.S. Department of Energy ( U.S. National Nuclear Security Administration)
AfricaArray researchers are working to identify more accurately the origin time, location and cause of small seismic events associated with three deep South African gold mines. The researchers are building event databases from three sources: seismic-monitoring networks within the mines, at the surface around the mines and from AA observatories in South Africa and surrounding countries at distances up to several hundred kilometers from the mines. Pinpointing where an event occurred enables researchers both to better understand why it occurred and characterize the seismic activity.
In 2007—the first year of the project—more than 10,000 mining-related events were catalogued and recorded. Data from those events are unique because the events span several orders of magnitude, occurred at a range of depth and had varying causes from pillar collapses and rock bursts to slips on faults.
Besides additional data gathering, researchers also have begun using the datasets to define and calibrate seismic propagation characteristics, investigate the effects of depth on events and develop categories of event sources such as shear slips, pillar collapse, explosions. AA researchers also are defining regional phase ratios that will enable discrimination between the different source categories and comparison between those events and mining events in Scandinavia and the western United States. This will improve U.S. agencies’ capabilities of identifying and monitoring underground nuclear tests.
Delineating the southern margin of the Congo Craton
Principle Investigators: Andrew Nyblade, Paul Dirks, Ray Durrheim
Collaborators: National Meteorological Institute of Angola (INAMET)
Sponsors: BHP Billiton, DeBeers and Rio Tinto
The structure of the Congo Craton is poorly known, which impedes efforts to explore for mineral resources in large parts of central and southern Africa and also limits our understanding of African tectonics. In this project, the southern margin of the Congo Craton, which lies somewhere between western Angola and central Botswana, is being investigated by using data from a network of seismic stations in Angola, Namibia and Botswana to estimate lithosphere thickness. Determining the southern edge of the Congo Craton is of particular interest to companies prospecting for diamonds.
This research project includes the building of new seismic monitoring stations in Angola, Botswana, and Namibia and the training of students and technicians to use the data recorded by those stations. AfricaArray graduate students, for instance, are using the data to investigate the lithospheric structure of the southwest Congo Craton as well as the seismic velocity and attenuation structure of the crust and uppermost mantle beneath the western half of southern Africa. The project also is training geophysicists and technicians at INAMET and at Agostinho Neto University (Angola) in operating and maintaining the Angolan seismic network.
The seismic data also are being used to compile the first-ever modern seismic hazard map for Angola, which experienced a series of earthquakes in 2002 to2004.