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An analysis of some local earthquake phases originating near the afar triple junction

GEOPHYSICAL OBSERVATORY HAILE SELASSIE I UNIVERSITY, P. O. BOX 1176, ADDIS ABABA, Ethiopia

Abstract

A study of Pn, Sn and Lg phases from 86 earthquakes which have occurred within 12.4° of WWSS station AAE is presented. Travel-time curves for each phase have been determined, and the corresponding seismic velocities have been deduced from them. Velocities of 7.95 km/sec and 4.29 km/sec were found for Pn and Sn respectively. Two different Lg velocities were found: 3.50 km/sec for ray paths between Uganda and Addis Ababa, and 3.73 km/sec for ray paths in the Red Sea and northern Ethiopia. The travel-time curves also allow an upper limit of 48 km to be placed on the crustal thickness under AAE.

Regional variations in the efficiency of propagation of Sn and Lg are discussed. Efficient propagation of Lg from epicenters near the center of the Red Sea suggests that not all of the Red Sea floor is pure oceanic crust. Sn is not propagated across northern Afar, suggesting that a gap occurs in the lithosphere there, but it is propagated efficiently across much of southern Afar. Finally, the seismic parameters deduced here indicate the existence of a widespread region of high temperature, low velocity, low density upper-mantle material underlying the Afar triple junction and the surrounding regions.