SEISMIC ANISOTROPY BENEATH THE AFAR DEPRESSION, ETHIOPIA, AND ADJACENT AREAS: IMPLICATIONS FOR MANTLE FLOW

Shear-wave splitting is a robust tool to infer the direction and strength of seismic anisotropy in the lithosphere and underlain asthenosphere. Previous studies of shear-wave splitting measurements obtained in the Afar Depression and adjacent areas concluded that either Precambrian suture zones or vertical magmatic dikes are mostly responsible for the observed azimuthal anisotropy. Here we report results of a systematic analysis of teleseismic shear-wave splitting using all the available broadband seismic data recorded in the Afar Depression, Main Ethiopian Rift (MER), and Ethiopian Plateau. The approximately 600 pairs of well-defined measurements reveal a systematic azimuthal dependence of splitting parameters with a 90-degree periodicity in the Afar Depression, where the splitting parameters fit well with a two-layer model of anisotropy. The top layer is characterized by a relatively small (0.65 s) splitting time and a WNW fast direction that can be attributed to magmatic dikes within the lithosphere, and the lower layer has a larger (2.0 s) splitting time and a NE fast direction. Using the spatial coherency of the splitting parameters obtained in the MER and on the Ethiopian Plateau, we estimated that the optimal depth of the source of anisotropy is 300 km, i.e., in the asthenosphere.The spatial and azimuthal variations of the observed anisotropy, when combined with seismic tomography and other results, can best be explained by a rift-parallel flow in the asthenosphere beneath the entire study area with small contributions from rift-parallel magmatic dikes.