Ambient Seismic Noise Tomography and 3-D Shear Wave Velocity Inversion in the Afar Depression and the Conjugate Margins of the Southern Red Sea

The Afar Depression is a unique place on Earth where continental break-up and ongoing rifting process can be studied using subaerial geophysical technics. Using data collected by more than 100 stations deployed in the course of 6 spatially and temporarily overlapping seismic projects in Ethiopia, Yemen and Djibouti, in addition to data from global and regional seismic stations in the surrounding areas, we focus on the evolution of the crustal and uppermost mantle structures over the last 25 million years within the Afar Depression but also on the conjugate margins of the southern Red Sea. To do so, we performed an ambient noise tomography to obtain (7 to 35 sec) 2-D phase velocity maps of western Yemen and Afar. The phase velocity measurements are then used to invert 3-D shear wave velocity for the upper 50 km of the lithosphere. The 2-D phase velocity results correlate well with the known surface and sub-surface geological and tectonic units of the rift. Low velocity anomalies are concentrated along the axial volcanic systems, rift flanks and sedimentary basins. The high velocity zones are associated with granitic intrusions and are interpreted as the presence of underplating material. The 3-D shear wave velocity results shows that the crustal thickness decreases from the Ethiopian uplifted plateaus (30-45 km), to Central Afar (~24-27 km) with a minimum of ~17 km beneath the southern part of the Danakil depression. Neither flat Moho nor sharp discontinuity could be identified within the upper 50 km depth beneath Afar. However, a smooth transition from the lower crust velocity (~3.6±0.1 km/s) to an anomalous upper part of the Mantle (~3.8±0.1 km/s) is evidenced. On the western Yemen side of the rift, the crustal thickness decreases from ~35 km beneath the plateau to ~22-26 km below the coastal plain, reaching westward~10 km beneath the Red Sea axial trough north of 14°N.