Lower mantle structure beneath Africa imaged by SKS-SKKS differential splitting and travel time delays

Seismic observations of the lowermost mantle show that this deep part of the earth's interior is highly heterogeneous. This includes the presence of two large low shear velocity provinces (LLSVPs), and the presence of seismic anisotropy, particularly at the margins of the LLSVPs. Here, we investigate the lowermost mantle in the vicinity of the African LLSVP using observations from over 500 stations. This data set comprises all publicly available permanent stations in Africa and temporary experiments since 1990. This enables us to map the spatial distribution of the anisotropic and isotropic regions of the lowermost mantle with unprecedented resolution and will eventually help to infer deep mantle flow patterns and mineralogical properties.

To derive anisotropic properties, we analyze shear-wave splitting discrepancies of teleseismic SKS and SKKS phases for the same station-event pairs. Though these phases are traditionally used to study anisotropy in the upper mantle and infer past and present mantle flow processes, discrepancies in their splitting measurements can be exploited to resolve lowermost mantle anisotropy because of their divergent pathways in the lowermost mantle. This approach has previously been used to determine seismic anisotropy at the edge of the LLSVPs. We find evidence for differential SKS-SKKS splitting in our data set, indicating the presence of lowermost mantle anisotropy. Additionally, we observe large differences between expected and observed arrival times of SKS and SKKS phases. We use these to quantify the time delay caused by the low velocity zone beneath Africa, which is much more pronounced than suggested by tomography models.