Vertical deformation in the Main Ethiopian Rift: levelling results in its northern part, 1995-2004

A levelling line consisting of 43 benchmarks was established between the towns of Wolerchiti and Metehara in the northern part of the Main Ethiopian Rift in July 1995 by the Geophysical Observatory of Addis Ababa University and the Ethiopian Mapping Authority. The measurement was repeated in November 2003 on 30 of the surviving benchmarks. In both epochs of measurement, the standard accuracy attained is +/- 4 mm root K corresponding to a first order levelling, where K is the inter-benchmark distance in kilometres. The line crosses the northern and southern parts of the Nazret (Boset-Kone) and Sabure (Fantale-Dofen) magmatic segments respectively where 80% of rift deformation is believed to be localized. In eight years, interval height differences ranging from +3 mm to - 22 mm are found along the line with the maximum subsidence rate of 2.8mm a(-1) corresponding to the Kone-Gariboldi volcanic complex in the northern part of the Nazret magmatic segment. On the other hand, at the eastern end of the line, despite the existence of large fissures and an expanding lake in the vicinity suggesting possible significant subsidence, on the contrary relatively small vertical deformation is found. The strong subsidence measured at the Kone-Gariboldi volcanic complex is interpreted to be due to remnant processes of subsidence at these volcanic centres following withdrawal of magma in the recent past. Regarding the eastern end of the line where Lake Beseka is located, the result is particularly important in verifying that the rapid expansion of the lake associated with elevation increase of 40 cm a(-1) could not be attributed to tectonic subsidence or uplift in the region.