5 Years of GPS Observations in the Afar Triple Junction Area

A. Walpersdorf, C. Vigny, J. Ruegg, P. Huchon, L. M. Asfaw, and S. Al Kirbash

In November 1991, a network of about 30 GPS sites was measured in Djibouti (Somalian plate). Additional points were also measured in Yemen (Arabia) and in Ethiopia (Nubia). Since then, a few points of this network were re-occupied several times (from January 1993 to March 1995) for short durations. The present day data set include measurements from 1991 to 1995 on the large-scale baselines (Arta, Djibouti - Sana'a, Yemen - Addis Abeba, Ethiopia) and repeat measurements on more local baselines in Djibouti. The station of Arta in Djibouti shows very large displacements: locally representative (as attested by a local tie), but different from other stations located on the stable zone outside the overlap area of the Asal-Tadjoura rifts. This motion does not seem to be linear with time, but rather suggests postseismic deformation (related to the Arta earthquake of March 5, 1992, magnitude 5.4). Except for the episodic Arta displacement, coherent motions of three stations in Southern Djibouti with respect to Yemen are observed, determining a more stable zone south of the overlap area. The observed opening of the Afar triple junction yields at least the NUVEL1-A estimate of 16 mm/a between Yemen and Djibouti.
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