Lower-Crustal Seismicity on the Eastern Border Faults of the Main Ethiopian Rift

Lower-crustal seismicity is commonly observed in continental rift zones despite the crust at such depths being ductile enough to prohibit brittle failure. The source of such deep seismicity across the East African Rift remains an outstanding question. Here we present analysis of an isolated cluster of lower-crustal earthquakes located on the eastern border faults of the Main Ethiopian Rift, near the Corbetti caldera. Lower-crustal earthquakes have not previously been observed in this area. Phase arrival times were determined using an automated picking approach based on continuous wavelet transform and statistical changepoint detection methods. We overcome misinterpretations from large hypocenter depth errors by considering mixture distributions for all events and their associated uncertainties. These mixture distributions represent probability density functions of any event occurring at a given depth. The mixture distribution mode for a variety of different velocity models and error parameters remained stable at a depth of 28–32 km, with the vast majority of maximum likelihood estimates for individual hypocenters located at depths of 25–35 km. Most events occur over a 2 month period, with 90% of cumulative seismic moment occurring during March and April 2012. The ephemeral and localized nature of this seismicity, combined with low event magnitudes and regional hydrothermal/magmatic activity, suggests that these lower-crustal events are likely related to fluid or magmatic processes. Plausible mechanisms include the movement of magma and/or exsolution of volatiles at depth causing transient high strain rates and pore fluid pressures that induce seismicity.