Attenuation and Velocity Tomography of the Northern East African Rift

Abstract

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 ($P$) and shear-wave ($S$) velocity ($V$) and attenuation ($q$) 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 ($V_P$, $V_S$, $q_S$) 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.