IGF



Master of Science Dissartation

Seismic structure and geodynamic model of Large Low Shear Velocity Provinces

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Agnieszka Płonka

dr hab. Leszek Czechowski

Wydział Fizyki UW

2013

Right above the core-mantle boundary beneath Africa and central Pacific, seismological observations have revealed regions of reduced shear velocity, referred as Large Low Shear Velocity Provinces (LLSVP). Their seismic structure, influence on the mantle dynamics and genesis is largely unknown. They likely correspond to dense, chemically distinct material accumulates on the core-mantle boundary.
To investigate the eastern border of the Pacific LLSVP we processed data from natural earthquakes using the IRIS catalogue and Wilber II tool. We calculated differential travel times between S diffracted and SKS arrivals, as well as S diffracted and synthetic seismograms calculated with respect to the PREM velocity reference model. Then we employed a simple ray-tracing algorithm to determine the velocity change in certain areas of the ray path. We calculated deviations from the PREM model for depths of range between 1000 and 2800 km, and the optimal LLSVP border. The results were compared with the data for the southern border of the African LLSVP.
In the geodynamic chapter we wanted to link seismological observations with results from numerical modeling. We used a two-dimensional model of mantle convection and calculated dense material distribution in the lower thermal boundary layer for different densities, Rayleigh number values and radiogenic heat productions. For certain density differences between the convecting layer material (the mantle) and dense material in the lowermost layer we obtained two dinstinct regions of accumulation that may correspond to the observed LLSVPs.


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