Thermo-Rheological Properties of the Ethiopian Lithosphere and Evidence for Transient Fluid Induced Lower Crustal Seismicity Beneath the Ethiopian Rift

Lower crustal earthquakes at plate boundaries and intraplate settings occur at depthwhere deformation is normally expected to occur in a ductile manner. Here we usethe available earthquake catalogs and compute theoretical predictions for a range ofconditions for the occurrence of lower crustal earthquakes beneath the Main EthiopianRift (MER) and adjacent north-western (NW) plateau. Yield strength envelops areconstructed using information on geothermal gradient, strain rate, and compositionconstrained by geophysical observations. Our models suggest that away from the MERbeneath the NW plateau the depth distribution of earthquakes in the lower crust is bestexplained by strong mafic lower crustal rheology and hydrostatic fluid pore pressureconditions. In the same region the effective elastic thickness is similar to seismogenicthickness showing that the lower crust has long-term strength and hence can physicallysupport brittle deformation. On the contrary, in the central MER the seismogenicthickness is much larger than the effective elastic layer thickness implying that thelower crust has no long-term strength. Here our models show that both hydrostatic andnear-lithostatic fluid pore pressures fail to explain the observed seismicity and instead acombination of near-lithostatic pore fluid pressure and transient high strain rate due tothe movement of fluids provide a plausible mechanism for the occurrence of seismicity inthe lower crust. Our interpretations are supported by occurrence of swarms of deepearthquakes beneath the MER, as opposed to more continuous background deepseismicity away from the rift. Using time-depth progression of earthquakes, we estimatepermeability values of 5.9 × 10-15 m2 and 1.8 × 10-14 m2 at lower crustal depth. Therange of permeability implies that seismicity can be induced by pore-pressure diffusion,likely from fluids sourced from the mantle that reactivate preexisting faults in the lowercrust. Our thermo-rheological models explain the first order differences in lower crustalearthquakes both directly beneath and outboard of the rift valley.