Inferring the Sharpness and Structure of the African LLVP Boundaries from Slowness-Backazimuth Measurements of Multipathing
Abstract
The Large Low-Velocity Provinces (LLVPs) may be of (potentially primordial) thermochemical origin , purely thermal features, or some combination thereof. Determining what LLVPs are has implications on whole mantle dynamics and the Earth's evolution from its primordial state, but current observations are not enough to constrain their key properties. Seismic tomography helps constrain the location, morphology and approximate velocity relative to the surrounding mantle, whilst regional seismic travel time and 'multipathing' studies give details of the location and sharpness of their boundaries. Multipathing occurs when a wave is incident on a sufficiently large velocity gradient. This causes different sections of the wave to move at different speeds and be diffracted at the boundary, therefore arriving at the station at different times and directions. Typically, multipathing is identified using the waveform only and properties such as the direction and incidence angle are not analysed.
In this study, we use array seismology to measure the backazimuth (direction) and absolute slowness (incidence angle) of the potentially multiple arrivals and identify multipathing. We apply the method over several frequency bands to infer the sharpness of the boundaries. Using a dataset of SKS and SKKS waves sampling the African LLVP, we measure backazimuth deviations of up to 20°, absolute slowness deviations of up to 0.8 s/° and detect multipathing in approximately 15% of our observations. Analysing our observations spatially implies: the African LLVP boundary varies in sharpness; seismically fast and slow regions together contribute to multipathing; and multipathing occurs at several depths. We conduct finite-frequency forward modelling by manipulating the velocity perturbations of published tomography models to constrain the conditions required to reproduce our data. Although the presence of velocity variations in the upper mantle appears to reduce the detectability of multipathing, it does not cause it; rather the lower mantle is the cause of the signals. We find preliminarily that our observations are matched most closely when the tomographic gradients are trebled below 1000 km. This suggests gradients of 0.7 δVs(%) /° present to 1400 km depth are required to explain the observed seismically multipathed wavefield.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMDI41C0002W
- Keywords:
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- 3924 High-pressure behavior;
- MINERAL PHYSICS;
- 3621 Mantle processes;
- MINERALOGY AND PETROLOGY;
- 8124 Earth's interior: composition and state;
- TECTONOPHYSICS;
- 8125 Evolution of the Earth;
- TECTONOPHYSICS