Eruption frequency and magnitude in a geothermally active continental rift: The Bora-Baricha-Tullu Moye volcanic complex, Main Ethiopian Rift

Many Quaternary silicic volcanoes in the Main Ethiopian Rift pose a potential risk due to the poorly known eruptive histories of the volcanoes in combination with a high population density. In this study we provide new constraints on the Late Pleistocene-Holocene eruptive history of the Bora-Baricha-Tullu Moye (BBTM) volcanic complex located in the central portion of the Main Ethiopian Rift (MER). BBTM constitutes three main silicic edifices (i.e. Bora, Baricha and Tullu Moye) and numerous smaller vents (including Oda and Werdi). Tephra deposits from these vents are several centimetres to meters in thickness in currently densely populated regions and where geothermal development is taking place. We present new field observations in addition to physical, petrographic, geochemical and geochronological data. BBTM experienced at least 27 explosive eruptions, of varying magnitude, in the last ca. 100 ky. The two oldest tephra deposits in our compiled stratigraphy are associated with large-magnitude, and possibly caldera-forming eruptions. The youngest of these (Meki) occurred at 107.7 ± 8.8 (2σ) ka, which makes it the youngest caldera-forming eruption identified in the Central MER so far. During the post-caldera stage, BBTM underwent at least 25 eruptions sourced from the Baricha (9 eruptions), Bora (3), Oda (8), Werdi (3) and Tullu Moye (2) edifices. The return period of explosive activity in BBTM is thus at least one moderate-to-large explosive eruption every 4000 yr. Well-exposed units have estimated eruption magnitudes (M) that are 4 to 5, while smaller-scale eruptions reach up to 2.5 and are exclusively preserved near the Tullu Moye vent. The tephra was dispersed up to 20 km from the volcanic complex suggesting that more than one hundred thousand people could be exposed to tephra fall and pyroclastic density current hazards from future of similar-magnitude eruptions in this area.