Abstract
Observations of ground deformation in East Africa have been fundamental for unveiling the tectonics of continental rifting, assessing the seismic and volcanic hazard to development, and identifying geothermal resources. Here we investigate the active natural and anthropogenic processes in the Tendaho Graben, Afar using Interferometric Synthetic Aperture Radar (InSAR) collected by the Envisat satellite in 2004–2010. We used the Poly-Interferometric Rate And time series Estimation (π-RATE) method to calculate displacement in satellite line-of-sight, and a least-square inversion to decompose the line-of-sight displacement into vertical and rift perpendicular components. We observe two zones of deformation: a 20 km wide circular region of subsidence located 10 km northeast of the town of Semera with a maximum displacement rate of ∼5 cm/yr; and elongated zone (50 km) of subsidence in the area of the geothermal prospect, maximum rate of ∼4 cm/yr. The temporal characteristics of subsidence varies between these zones, with an increase in subsidence rate observed in the circular region in August 2008. We used a Bayesian inversion to find the best fitting source models and compared this to locations of seismicity and other geophysical observations. The pattern of deformation is consistent with a combination of magmatic and geothermal processes, but there does not appear to be a direct link to a sequence of dyke intrusions during 2005–2010 at Manda Hararo graben ∼60 km away, but dynamic stress changes or deep crustal flow could account for the observations.
Highlights
IntroductionNumerous natural and anthropogenic processes can cause surface deformation, including magmatic processes (e.g. Wright et al, 2006; Hamling et al, 2009), volcanic processes (e.g. Biggs et al, 2011; Lu et al, 2010), hydrothermal circulation (e.g. Kwoun et al, 2006; Vilardo et al, 2010; Biggs et al, 2011), fault processes (e.g. Hussain et al, 2016; Hamling et al, 2017) and water pumping from, and recharge of, water aquifers (e.g. Galloway et al, 1998; González et al, 2012)
The Tendaho Dam is built on the fault scarp of Tendaho Goba’Ada Discontinuity (TGD) (Fig. 1) which is the northern end of Main Ethiopian Rift (MER) (Ayele et al, 2015).This study was initially designed to investigate the loading effects caused by the Tendaho Dam, which was known to have triggered local seismicity (Ayele et al, 2015)
The magnitude of the signal in both deformation regions is much smaller than measured in the Manda Hararo magmatic segment, but is comparable to that observed at silicic volcanoes and geothermal systems within the Main Ethiopian Rift (e.g. Biggs et al, 2011; Hutchison et al, 2015) and in the Kenyan Rift (Biggs et al, 2009, 2016a; Robertson et al, 2016)
Summary
Numerous natural and anthropogenic processes can cause surface deformation, including magmatic processes (e.g. Wright et al, 2006; Hamling et al, 2009), volcanic processes (e.g. Biggs et al, 2011; Lu et al, 2010), hydrothermal circulation (e.g. Kwoun et al, 2006; Vilardo et al, 2010; Biggs et al, 2011), fault processes (e.g. Hussain et al, 2016; Hamling et al, 2017) and water pumping from, and recharge of, water aquifers (e.g. Galloway et al, 1998; González et al, 2012). Despite the tectonic and volcanic activity in this area, it is rapidly developing, and major infrastructure development projects include the recently-completed Tendaho Dam (Ayele et al, 2015) and the Tendaho geothermal prospect (Aquater, 1996; Gianelli et al, 1998; Battistelli et al, 2002; Didana et al, 2014). We use InSAR to investigate the spatial and temporal characteristics of deformation within the Tendaho geothermal prospect. Understanding the spatially complex pattern of surface deformation is important for unveiling the tectonics of the region, for assessing the seismic and volcanic hazard to development, and for identifying geothermal resources
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