Along- and cross-axis variations in crustal rift structure observed in the Main Ethiopian Rift

We estimate a 3D shear wave velocity model for the crust of the Main Ethiopian Rift (MER) using the technique of ambient noise tomography. The obtained model includes the entire central MER (CMER) and much of the southern MER (SMER), and therefore provides new information on crustal structure along the MER. In the CMER, our model shows higher velocities to the south of the southernmost Quaternary magmatic center along the Wonji Fault Belt (eastern part of the CMER), and the persistence of low velocities through the crust beneath the Silti-Debre zeit Fault Zone (western part of the CMER). This is consistent with the magmatic plumbing model for the MER proposed by geochemical studies. With respect to the along-axis direction, higher velocity in the SMER indicates that the mid and lower crust is not currently being modified by magmatism. In contrast, a prominent lower velocities are shown beneath the Boset-Kone magmatic segment in the NMER, where the significantly less seismicity, the increase in crustal anisotropy, and the overall higher crustal Vp/Vs ratios were observed. This region also coincides with the present-day intense zone of melt extraction from asthenosphere, which is expected by the lowest velocity in the upper mantle and the maximum SKS delay time. These evidences show that the along-axis variation can be attributed to the discontinuous development of the MER parts, which show that rift initiation in the Afar and the SMER could be at least 10 Ma older than in the CMER and the NMER.