Attenuation and Velocity Tomography of the Northern East African Rift

• Published in: Journal of geophysical research. Solid earth Vol. 130; no. 2
• Main Author: Eilon, Z. C.
• Format: Journal Article
• Language: English
• Published: 01.02.2025
• Subjects: attenuation tomography; continental rift; East African Rift; Main Ethiopian Rift; seismic attenuation
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2024JB030417

The northern East African Rift, including the Main Ethiopian Rift (MER) and Afar, have been the focus of decades of seismological investigations and dense broadband coverage. Sometimes invoked as an archetype for a narrow continental rift, the confined surficial extension here stands in contrast to geophysical indicators of an asymmetric, broad swath of perturbed mantle. Thermodynamic conditions in this region are challenging to pin down, with melt, temperatures, and perhaps volatiles playing a part in both dynamics and controlling seismic observables. This study presents compressional (P $P$) and shear‐wave (S $S$) velocity (V $V$) and attenuation (q $q$) teleseismic tomography of the MER, from rift to flank. This involves a new methodology for measurement and quality control of differential t* for teleseisms, avoiding extrinsic focusing. This is the most comprehensive body wave travel time and attenuation data set collected in the MER and Afar to date, and the only 3‐D attenuation tomography model of a continental rift. Together, the well‐resolved co‐located models (VP ${V}_{P}$, VS ${V}_{S}$, qS ${q}_{S}$) help constrain the physical conditions within the rift through Bayesian analysis that makes explicit parameter trade‐offs. The rift is highly asymmetric, with a sharp eastern boundary but extensive low velocities, high attenuation, and implied melt organization throughout much of the western Ethiopian Plateau, including coincident with volcanism around Lake Tana. A deep‐seated plume is connected to shallow rifting. Attenuation and velocity both imply a concentration of extensional strain along the western edge of the Afar triangle. Plain Language Summary The continent of Africa is splitting apart along the East African Rift. The northern section of this rift, the Main Ethiopian Rift, is narrow and bracketed by steep faults, suggesting the continent splits in a relatively clean break, assisted by upwelling magma. This study presents 3‐D models of the breaking tectonic plate and the mantle underneath it. Each model covers an identical area, but looks at the rift in a different way: two models capture different aspects of the speed of seismic wave propagation, and another captures (for the first time, here) how seismic waves lose energy. This latter is often not measured, because it is difficult to calculate. Any one of these models is subject to ambiguities in interpretation of physical parameters we care about (e.g., temperature, where there is magma), but together they offer powerful new insights into the rift. The rift is not everywhere narrow beneath the surface. The western side of the rift has a highly degraded tectonic plate, as deep magma seems to be broadening the rifting from the “bottom up.” Meanwhile in the north, effects of rifting appear to be concentrating in the west of the Afar triangle. Key Points P‐ and S‐wave teleseismic attenuation and travel time are measured within and on the flanks of the Main Ethiopian Rift and Afar Congruent 3‐D velocity and attenuation tomography depicts upper mantle structure, allowing inversion for temperature, melt, and grain size The rift is hot and asymmetric, with strain concentration in west Afar and strain diffusion in the western Ethiopian Plateau