Transient fluid induced lower crustal earthquakes beneath the Ethiopian Rift: Constraints from thermo-rheological modelling

Abstract:

Lower crustal seismicity in the Main Ethiopian Rift (MER) and north-western (NW) plateau is observed
at depths where the temperature is too high to allow brittle failure. Here we use the available earthquake
catalogues and compute theoretical predictions for a range of conditions for the occurrence of lower crustal
earthquakes beneath the MER and adjacent northwestern plateau. Yield strength envelops are constructed using information on geothermal gradient, strain rate and composition constrained by geophysical observations. Our models suggest that away from the MER beneath the NW plateau the depth distribution of earthquakes in the lower crust is best explained by strong mac lower crustal rheology and hydrostatic fluid pore pressure conditions. In the same region the effective elastic thickness is similar to seismogenic thickness showing that the lower crust has long-term strength and hence can sustain brittle deformation.

On the contrary, in the central MER the seismogenic thickness is much larger than the effective elastic
layer thickness implying that the lower crust has no long-term strength. Here our models show that both
hydrostatic and near-lithostatic fluid pore pressures fail to explain the observed seismicity and instead high
pore fluid pressure and transient strain rate due to the movement of fluids provide a plausible mechanism
for the occurrence of seismicity in the lower crust. Using time-depth progression of earthquakes, we estimate permeability values of 5.9x10^-15 m² and 1.8x10^-14 m² at lower crustal depth. The range of permeability implies that seismicity can be induced by pore-pressure diffusion, likely from fluids sourced from the mantle that reactivates preexisting faults in the lower crust. Our thermo-rheological models explain the first order differences in lower crustal earthquakes both directly beneath and outboard of the rift valley.