Unusually deep earthquakes occur beneath rift segments with and without surface expressions of magmatism in the East African Rift system. The Tanganyika rift is part of the Western rift and has no surface evidence of magmatism. The TANG14 array was deployed in the southern Tanganyika rift, where earthquakes of magnitude up to 7.4 have occurred, to probe crust and upper mantle structure and evaluate fault kinematics. Four hundred seventy-four earthquakes detected between June 2014 and September 2015 are located using a new regional velocity model. The precise locations, magnitudes, and source mechanisms of local and teleseismic earthquakes are used to determine seismogenic layer thickness, delineate active faults, evaluate regional extension direction, and evaluate kinematics of border faults. The active faults span more than 350 km with deep normal faults transecting the thick Bangweulu craton, indicating a wide plate boundary zone. The seismogenic layer thickness is 42 km, spanning the entire crust beneath the rift basins and their uplifted flanks. Earthquakes in the upper mantle are also detected. Deep earthquakes with steep nodal planes occur along subsurface projections of Tanganyika and Rukwa border faults, indicating that large offset (≥5 km) faults penetrate to the base of the crust, and are the current locus of strain. The focal mechanisms, continuous depth distribution, and correlation with mapped structures indicate that steep, deep border faults maintain a half-graben morphology over at least 12 Myr of basin evolution. Fault scaling based on our results suggests that M > 7 earthquakes along Tanganyika border faults are possible.

Corbetti is currently one of the fastest uplifting volcanoes globally, with strong evidence from geodetic and gravity data for a subsurface inflating magma body. A dense network of 18 stations has been deployed around Corbetti and Hawassa calderas between February 2016 and October 2017, to place seismic constraints on the magmatic, hydrothermal and tectonic processes in the region. We locate 122 events of magnitudes between 0.4 and 4.2 using a new local velocity model. The seismicity is focused in two areas: directly beneath Corbetti caldera and beneath the city of Hawassa. The shallower 0-5 km depth below sea level (b.s.l.) earthquakes beneath Corbetti are mainly focused in EW-to NS-elongated clusters at Urji and Chabbi volcanic centres. This distribution is interpreted to be mainly controlled by a northward propagation of hydrothermal fluids away from a cross-rift pre-existing fault. Source mechanisms are predominantly strike-slip and different to the normal faulting away from the volcano, suggesting a local rotation of the stressfield. These observations, along with a low Vp/Vs ratio, are consistent with the inflation of a gasrich sill, likely of silicic composition, beneath Corbetti. In contrast, the seismicity beneath Hawassa extends to greater depth (16 km b.s.l.). These earthquakes are focused on 8-10 km long segmented faults, which are active in seismic swarms. One of these swarms, in August 2016, is focused between 5 and 16 km depth b.s.l. along a steep normal fault beneath the city of Hawassa, highlighting the earthquake hazard for the local population.

Imaging the lithosphere is key to understand mechanisms of extension as rifting progresses. Continental rifting results in a combination of mechanical stretching and thinning of the lithosphere, decompression upwelling, heating, sometimes partial melting of the asthenosphere, and potentially partial melting of the mantle lithosphere. The northern East African Rift system is an ideal locale to study these processes as it exposes the transition from tectonically active continental rifting to incipient seafloor spreading. Here we use S‐to‐P receiver functions to image the lithospheric structure beneath the northernmost East African Rift system where it forms a triple junction between the Main Ethiopian rift, the Red Sea rift, and the Gulf of Aden rift. We image the Moho at 31 ± 6 km beneath the Ethiopian plateau. The crust is 28 ± 3 km thick beneath the Main Ethiopian rift and thins to 23 ± 2 km in northern Afar. We identify a negative phase, a velocity decrease with depth, at 67 ± 3 km depth beneath the Ethiopian plateau, likely associated with the lithosphere‐asthenosphere boundary (LAB), and a lack of a LAB phase beneath the rift. Using observations and waveform modeling, we show that the LAB phase beneath the plateau is likely defined by a small amount of partial melt. The lack of a LAB phase beneath the rift suggests melt percolation through the base of the lithosphere beneath the northernmost East African Rift system.