Confidential manuscript submitted to JGR: Solid Earth Lower Crustal Seismicity on the Eastern Border Faults of the Main Ethiopian Rift

19 Lower crustal seismicity is commonly observed in continental rift zones despite the crust at such depths 20 being ductile enough to prohibit brittle failure. The source of such deep seismicity across the East African Rift 21 remains an outstanding question. Here we present analysis of an isolated cluster of lower crustal earthquakes located 22 on the eastern border faults of the Main Ethiopian Rift, near the Corbetti caldera. Lower crustal earthquakes have 23 not previously been observed in this area. Phase arrival times were determined using an automated picking approach 24 based on continuous wavelet transform and statistical changepoint detection methods. We overcome 25 misinterpretations from large hypocentre depth errors by considering mixture distributions for all events and their 26 associated uncertainties. These mixture distributions represent probability density functions of any event occurring 27 at a given depth. The mixture distribution mode for a variety of different velocity models and error parameters 28 remained stable at a depth of 28 – 32 km, with the vast majority of maximum likelihood estimates for individual 29 hypocenters located at depths of 25 – 35 km. Most events occur over a two-month period, with 90% of cumulative 30 seismic moment occurring during March and April 2012. The ephemeral and localized nature of this seismicity, 31 combined with low event magnitudes and regional hydrothermal / magmatic activity, all suggest that these lower 32 crustal events are likely related to fluid or magmatic processes. Plausible mechanisms include the movement of 33 magma and/or exsolution of volatiles at depth causing transient high strain rates and pore fluid pressures that induce 34 seismicity. 35