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James Hammond, University of Bristol (United Kingdom)
J-Michael Kendall, University of Bristol (United Kingdom)
Doug Angus, University of Bristol (United Kingdom)
James Wookey, University of Bristol (United Kingdom)
Derek Keir, University of Leeds (United Kingdom)
Cindy Ebinger, University of Rochester (United States)
Atalay Ayele, University of Addis Ababa (Ethiopia)
Graham Stuart, University of Leeds (United Kingdom)
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Following the 2005 Dabbahu rifting event in Afar, 9 broadband seismometers were installed around the newly active rift segment. These recorded more than one year of continuous data. Additionally a new array of 38 stations in a multi-national, multi-institutional collaboration are currently deployed, of which we show up to a year of data. Shear-wave splitting observed in core phases (SKS/SKKS) shows considerable variability across the array. In Afar abrupt changes in the fast direction of 65° over distances of ∼30km are present in close proximity to the proposed Red Sea - Gulf of Aden - East Africa Rift triple junction, mimicing the strike of the plate boundaries. Stations on the rift margin, further from the recent activity show smoothly varying fast directions, aligning parallel to the border faults. The observations in Afar support previous work in the Main Ethiopian Rift (MER), where fast directions change abruptly from being rift parallel on the rift flanks to magmatic-segment parallel in the rift valley, with δt largest on the rift flanks. This was interpreted in terms of melt-induced anisotropy and support ideas of magma-assisted rifting in continental regions. The abrupt change in splitting parameters over small lateral distances suggests that the source of anisotropy is shallow. To further constrain the location of the anisotropy, and study the influence of the transition from rift margin to rift valley on shear-wave splitting results, we model finite-frequency waveforms for a suite of model representations of the rift zone. This allows us to determine the lateral and vertical extent of the melt-induced anisotropy. The results show how a simple model with two regimes of anisotropy can explain the variability of shear-wave splitting results across the rift zone, in both delay time and shear-wave polarisation, over short length scales.
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