Monitoring deformation in Afar, Ethiopia, using Envisat Wide-Swath Interferometry
Wright, Tim¹; Pasquali, Paolo²; D'Aria, Davide³; Monti Guarnieri, Andrea⁴; Hamling, Ian^1
1University of Leeds; ²Sarmap s.a.; ³ARESYS s.r.l.; ⁴Electronic Department - Politecnico di Milano

The diverging Arabian, Nubian and Somalian plates meet at a triple junction in the Afar depression, Northern Ethiopia. In September 2005, a 60 km segment of the Arabia-Nubia plate boundary opened by up to 8 m in the course of around 3 weeks, causing more than 160 earthquakes and a small volcanic eruption. More than 2 km³ of magma was injected into a volcanic dyke in the upper 10 km of the crust along the entire rift segment. This was the largest rifting episode to have occurred above sea level since the 1970s, and the first in the era of satellite geodesy. We have used wide-swath (ScanSAR) ASAR data acquired systematically in this area before, during and after the rifting episode to form interferograms that reveal the pre-, co- and post-dyking deformation that occurred. The data were processed by phase-preserving focusing the ScanSAR acquisitions and performing ad-hoc filtering to minimise the impact on decorrelation due to de-synchronisation of the burst patterns of the acquisition pairs to be interferometrically combined. The interferograms show that Gabho volcano, at the northern end of the rift segment, was uplifting prior to the rifting episode, probably due to the injection of fresh basaltic material into a magma chamber 3-5 km beneath Gabho. The co-rifting interferograms reveal large deformation, consistent with data derived image mode ASAR interferograms, but of poorer quality in the near-rift area because of the extreme deformation gradients there. Rapid post-rifting deformation is revealed in the wide-swath interferograms, consistent with ongoing dyke injections into the rift segment and/or viscous relaxation in the lower crust and upper mantle. In addition, the data show evidence for rapid recharge at Gabho and Dabbahu volcanoes. Wide Swath interferometry offers the potential to monitor a large area like Afar routinely.

Wide Swath Interferogram showing the deformation that occurred during the September 2005 Afar rifting episode