Gravity Field Studies In fuel Main Ethiopian Rift and Southern Afar
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Date
1997-06
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Addis Abeba university
Abstract
All available data, over 3000 stations, obtained from the EIGS and the Geophysical
observatory (Dr. Abera's Ph.D work) were reduced to sea level with a uniform crustal
density of 2.67 gmicm3
• Effects of Bouguer masses were calculated applying the simple
Bouguer cOITection. Terrain correction was not applied in which case its effect was
treated as systematic error in computing the over all mean square elTor of the simple
Bouguer anomalies at each station. Theoretical gra\·ity field was computed by means of
the intemational gravity fonnula of 1967 (GRS67) and tied to the IGS};71. The accuracy
of the Bouguer anomaly at each station is calculated to about ± 2.5 mga!. The regional
and residual gravity field were calculated from the Bouguer anomaly map by mea~ of a
computer software.
The compiled Bouguer anomaly map shows a strong con'elation between the regional
pattern and topographic feature of the survey area. Owing to higher elevations, the
adjacent plateaus are marked by negative Bou"ouer anomalies. the minimum of magnitude
-270 mgals being located around Debreberhan for the study area. The maximu'm gravity
values occur in the Afar de]Jression conesponding to the relatively lower elevations. TIle
main Ethiopian rift including Afar is characterized by a relatively positive anomaly as
compared to the plateaus. The high on the rift accounts for the excess mass at depth on
which the crust lies relative to the plateaus where a mass deficiency is assumed.
Gravity values on the main Ethiopian rift are generally less than those in the Afar. TIils
accounts for the th. inning of crust material towards the .-\far along the rift, or it can equall.y
be explained as by a thickening of a denser material beneath the crust (probably the
anomalous mantle). The margins separating the rift £i'om the plateaus are marked by steep
~
gra\ity gradients with their magnitude' representing the slope of the escarpment. Steep
gradients characterize the transition of gra\.ity fidd from the plateaus to the rift floor
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