Teleseismic P -Wave Attenuation Along the East African Rift System: Strong Attenuation Within the Main Ethiopian Rift

Remarkably strong anomalies in seismic properties of the mantle beneath the East African rift are plausibly explained by elevated mantle potential temperatures and/or the presence of partial melt. Previous studies have attributed strong attenuation to elevated temperatures that possibly extend as deep as the mantle transition zone. Although petrologic work clearly infers elevated temperatures, the ∼140°C anomaly is less than at many other hotspots. The anomalies in seismic velocity are stronger than can be explained by this modest excess in temperature, and hence additional mechanisms such as melt are often invoked. We measure the attenuation of a large dataset of teleseismic P phases stretching from the Arabian Platform to the Kaapvaal craton to constrain the nature of seismic anomalies beneath this plate boundary. Anomalies in attenuation are modest along our nearly 6000 km long profile, with the strong exception of the Main Ethiopian Rift (MER). We show that the anomalies in seismic velocity and attenuation beneath the Afar Depression and MER are jointly explained best by a relatively thin layer of high attenuation, akin to other regions of rifting, which we attribute to either “pre-melting” relaxation or an effect of an in situ melt phase. Because our data are path-integrated, thickness and QP trade off directly, and a thicker, more weakly attenuating region spread over hundreds of kilometers is an admissible end member. The opposite occurs further south—rifting occurs along the edges of the Tanzania craton and beneath Lake Malawi without strong attenuation, including beneath the Turkana Depression where the asthenosphere conditions must differ from the MER.