Constraints on ground deformation processes at the Tulu Moye volcanic complex, Main Ethiopian Rift

Abstract

Tulu Moye is an actively deforming volcanic complex with a geothermal field in the Main Ethiopian Rift. We use InSAR between 2014 and 2022, integrated with other geophysical data, to investigate the temporal and spatial characteristics of the deformation signal in the area, and to model its source. Velocity maps and time series analysis show a deformation signal consistent with uplift at a velocity of up to 50 mm/yr in the satellite Line-of-Sight (LOS) in 2014–2017, then decreasing to 12 mm/yr until 2022. The centre of deformation is located about 10 km west of a main geothermal drilling site at Tulu Moye, between the Bora, Berecha, and Tulu Moye volcanoes, with a NW-SE elongation direction. Our best-fit model suggests that the deformation is caused by an 8.7 km by 1.2 km sill situated ∼7.7 km below the surface (∼5.9 km below sea-level), elongate in the N54°W direction and dipping S11°W, and experienced an average velocity of volume change of ∼8.9 × 106 m3/yr in 2014–2017. The surface projection of the sill overlaps with local transverse faults and hydrothermal manifestations. The sill is ∼1–2 km below clusters of microseismic swarms and a region of high resistivity, both indicating hydrothermal fluid flow. The location and geometry of the sill correlates with the upper edge of high conductivity interpreted as a zone of partial melt, and we therefore attribute the uplift at the Tulu Moye volcanic complex to inflow of magma in the sill. We also suggest that the transverse caldera rims faults may restrict magma flow, and also facilitate both vertical and lateral hydrothermal fluid flow.