Abstract
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Abstract; We investigated the primary mechanisms triggering the S-wave splitting of 7 unusually deep local earthquakes (between 50km and 60km) that originated in the lithosphere beneath the Rwenzori region, in attempt to develop an understanding of the relationship between anisotropic structures in the lithosphere and tectonic deformation processes. A total of 12 of 44 waveforms showed evidence of SWS on their polarization diagrams and were available for further analysis. The fast-wave direction (φ) and delay-time (δt) were estimated using the covariance matrix and the cross-correlation coefficient methods respectively. We observed a clockwise rotation of NW-SE and ~ENE-WSW trending φ-directions at stations deployed on the southward propagating Lake Albert rift segment. We related these directions to anisotropic fabric, probably lattice preferred orientation of preexisting olivine’s a-axis aligned with ESE absolute plate motion (APM) vector. At stations deployed away from the rift valley, we similarly observed WNW-ESE and NNW-SSE patterns of φ-directions which we associated to the shape preferred orientation of structures that were probably frozen in the lithosphere and are aligned with the present-day APM direction. We observed δt values ranging between 0.04±0.01s and 0.43±0.02s that significantly decrease with distance away from the rift axis which further supports the view that the anisotropy observed at stations deployed on the ESE propagating plate is related to aligned melt inclusions frozen into the surrounding lithosphere. We further observe that the δt increases linearly with ray-path length which could probably suggest a fairly uniform anisotropy between 50km to 60km depth.