Monitoring Fast-Growing Megacities in Emerging Countries Through the PS-InSAR Technique: The Case of Addis Ababa, Ethiopia (East Africa)

In the past three decades, the city of Addis Ababa has grown significantly in population, facilities, and infrastructure. The area involved in the recent urbanization is prone to slow natural subsidence phenomena that can be accelerated due to anthropogenic factors such as groundwater overexploitation and loading of unconsolidated soils. The main aim of this study is to identify and monitor the areas most affected by subsidence in a context, such as that of many areas of emerging countries, characterized by the lack of geological and technical data. The increasing availability of SAR data acquired by the Sentinel-1 mission around the world and the refinement of processing techniques that have taken place in recent years, allow one to identify and monitor the critical conditions deriving from the impressive recent expansion of mega cities such as Addis Ababa. In this work, the Sentinel-1 SAR images from 2014 to 2021 were processed through the PS-InSAR technique, which allows us to estimate the deformations of the earth's surface with high precision, especially in urbanized areas. The obtained deformation velocity maps and displace-ment time series have been validated using accurate second-order geodetic control points and compared with the recent urbaniza-tion of the territory. The results demonstrate the presence of areas affected by a vertical rate of displacement of up to 21 mm/year. These areas correspond to sectors that are most predisposed to subsidence phenomena due to the presence of recent alluvial deposits and have suffered greater anthropic pressure through the construction of new buildings and the exploitation of groundwater. Satellite interferometry techniques are confirmed to be a reliable tool for monitoring potentially dangerous geologi-cal processes, and in the case examined in this work, they represent the only way to verify the urbanized areas exposed to the risk of damage with great effectiveness and low cost, providing local authorities with crucial information on the priorities of interven-tion.