Seismicity of the Abaya geothermal field in the Ethiopian rift

Abstract:

The Ethiopian rift has formed over the past 18 Ma from the separation of the Nubian and Somalian plates. The extension is associated with seismic activity and active magma intrusion, primarily within the rift. Several of Ethiopia’s active magmatic centres have recently been targets for geothermal exploration and geothermal power plant development. The occurrence of shallow seismicity (generally shallower than 5km) at the magmatic centers is most commonly caused by fluid flow through open fractures in the hydrothermal system. The aim of this project is to characterise for the first time the background seismicity occurring at the Abaya geothermal prospect in the southern Ethiopian rift to identify regions of hydrothermal fluid flow through permeable fracture networks. In mid 2019, with the help of a UK Global Challenges Research Fund (GCRF) we installed five Guralp 6TD seismometers in the rift valley between Lake Hawassa to the north and Lake Abaya to the south. To date, we have constrained >50 hypocentres, which provide clear evidence for earthquake clusters along rift parallel fault networks. Magnitudes range from 1.0-4.1, while earthquake depths range from 0 to 20 km. The catalogue is complete above magnitude of 2.2 and has a b-value of approximately 0.9. Vp/Vs ratios >1.765 estimated from the Wadati diagram. The spatial association between seismically active faults, and the Abaya geothermal field, suggests the earthquakes may be induced by fluid migration.