Gravity Field Studies In Fuel Main Ethiopian Rift And Southern Afar

All available data, over 3000 stations, obtained from the EIGS and the Geophysicalrnobservatory (Dr. Abera's Ph.D work) were reduced to sea level with a uniform crustalrndensity of 2.67 gmicm3rn• Effects of Bouguer masses were calculated applying the simplernBouguer cOITection. Terrain correction was not applied in which case its effect wasrntreated as systematic error in computing the over all mean square elTor of the simplernBouguer anomalies at each station. Theoretical gra·ity field was computed by means ofrnthe intemational gravity fonnula of 1967 (GRS67) and tied to the IGS};71. The accuracyrnof the Bouguer anomaly at each station is calculated to about ± 2.5 mga!. The regionalrnand residual gravity field were calculated from the Bouguer anomaly map by mea~ of arncomputer software.rnThe compiled Bouguer anomaly map shows a strong con'elation between the regionalrnpattern and topographic feature of the survey area. Owing to higher elevations, thernadjacent plateaus are marked by negative Bou"ouer anomalies. the minimum of magnitudern-270 mgals being located around Debreberhan for the study area. The maximu'm gravityrnvalues occur in the Afar de]Jression conesponding to the relatively lower elevations. TIlernmain Ethiopian rift including Afar is characterized by a relatively positive anomaly asrncompared to the plateaus. The high on the rift accounts for the excess mass at depth onrnwhich the crust lies relative to the plateaus where a mass deficiency is assumed.rnGravity values on the main Ethiopian rift are generally less than those in the Afar. TIilsrnaccounts for the th. inning of crust material towards the .-far along the rift, or it can equall.yrnbe explained as by a thickening of a denser material beneath the crust (probably thernanomalous mantle). The margins separating the rift £i'om the plateaus are marked by steeprn~rngraity gradients with their magnitude' representing the slope of the escarpment. Steeprngradients characterize the transition of gra.ity fidd from the plateaus to the rift floor

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