5 years of GPS observations of the Afar triple junction area

In November 1991, a network of about 30 GPS sites were measured in Djibouti (Somalian plate). Additional points were also measured in Yemen (Arabia) a nd in Ethiopia (Nubia). Since then, a few points of this network were re-oc cupied several times (from January 1993 to March 1995) for short durations. The present day data set include measurements from 1991 to 1995 on the lar ge-scale baselines (Arta, Djibouti-Sana'a, Yemen-Addis Ababa, Ethiopia) and repeat measurements on more local baselines in Djibouti. The station of Ar ta in Djibouti shows very large displacements: locally representative (as a ttested by a local tie), but different from other stations located on the s table zone outside the overlap area of the Asal-Tadjoura rifts. This motion does not seem to be linear with time, but rather suggests postseismic defo rmation (related to the Arta earthquake of March 5, 1992, magnitude 5.4). E xcept for the episodic Arta displacement, coherent motions of three station s in Southern Djibouti with respect to Yemen are observed, determining a mo re 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 Ye men and Djibouti. (C) 1999 Elsevier Science Ltd. All rights reserved.