The drivers of Lower Crustal Earthquakes Along Magma-poor Portions of the East African Rift

Deep earthquakes along magma-poor sections of the East African Rift System (EARS) challenge our understanding of the controls on seismogenic thickness because they occur at greater depths and higher temperatures than the frictional-viscous transition zone in typical continental crust. Using a recently published catalogue of relocated earthquakes in southeastern Africa, we demonstrate that seismicity occurs down to the ~40 km deep Moho throughout the magma-poor southern EARS. We then explore the mechanisms that can account for this deep seismicity by combining 1D lithospheric strength profiles with available regional measurements of Moho thickness, the ratio of the crust's P- and S-wave velocities (VP/VS), and heat flow. As suggested by previous studies, we find that a mafic lower crustal composition, lower geothermal gradient, and/or high pore fluid pressure can locally facilitate the observed deep seismicity. However, there are sections of the southern EARS where the lower crust is felsic, dry, and warm, and in these cases, we propose that the embrittlement of the lower crust is best explained by strain localisation in space and time. This strain localisation could occur because fault and shear zones in magma-poor sections of the EARS are unusually narrow, or because strain is localised in space and time following large magnitude earthquakes.