Constraints on fault and crustal strength of the Main Ethiopian Rift from formal inversion of earthquake focal mechanism data

We evaluate the frictional strength of seismogenic faults in the Main Ethiopian Rift (MER) by inverting the available, well-constrained earthquake focal mechanisms. The regional stress field is given by - 119.6 degrees/77.2 degrees, 6.2 degrees/7.6 degrees, and 97.5 degrees/10.2 degrees for trend/plunge of sigma(1), sigma(2) and sigma(3), respectively agrees well with previous fault kinematic and focal mechanism inversions. We determine the coefficient of friction, mu, for 44 seismogenic faults by assuming the pore pressure to be at hydrostatic conditions. Slip on 36 seismogenic faults occurs with is mu >= 0.4. Slip on the remaining eight faults is possible with low mu. In general, the coefficient of friction in the MER is compatible with a value of mu of 0.59 +/- 0.16 (2 sigma standard deviation). The shear stresses range from 16 to 129 MPa, is similar to crustal shear stress observed in extensional tectonic regimes and global compilations of shear stresses from major fault zones. The maximum shear stress is observed in the ductile crust, below the seismologically determined brittle-ductile transition (BDT) zone. Below the BDT, the crust is assumed to be weak due to thermal modification and/or high pore fluid pressure. Our results indicate linearly increasing mu and shear stress with depth. We argue that in the MER upper crust is strong and deforms according to Coulomb frictional-failure criterion.