Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
continental deposits, constituted by well sorted sub-rounded gravel with sand having a silty matrix,
pyroclastic rocks constituted by tuffs, sand and gravel are present in the upper part of the basalt flows.
RECENT PYROCLASTIC EVENT
The stratigraphy of the area continues with an important horizontal pyroclastic episode constituted by several
types of pyroclastic rocks formed by ash, tuff, agglomerated, about 15m thick pebble gravel. A sand bench is
present inside the pyroclastic rocks. This materials is rounded and is composed of heterogeneous rocks. The
stratigraphic sequence ends with a thin light grey ignimbrite level with a strongly siliceous composition
(>70%), and large amounts of crypto-crystalline quartz. The thickness is approximately 10-12m. The
stratigraphic column of the pyroclastic event illustrates the relationships among the lithological types.
5.1.3 Seismology
The Gibe III dam is located in Ethiopia, in the vicinity (about 70 km) of the eastern branch of the east
African rift system. It is a region of diffuse seismic activity that suffered strong earthquakes throughout its
history, such as the August 25th 1906 (Ms=6.6 and 6.8), September 16th 1913 (Ms=6.2), July 14th 1960
(Ms=6.3) and October 25th 1987 (Ms=6.2 and Mw=6.2) events.
In order to perform regional seismic hazard diagnosis at the Gibe III dam site, it has been conducted an
analysis taking into account active faulting as identified on digital elevation models and satellite imagery.
Because the reliability of seismic hazard assessment strongly relies on a good understanding of the
geodynamics of a given region at different scales, the deterministic analysis of seismotectonics and
Quaternary tectonics of the east African rift region has been performed in several stages, from plate tectonics
to seismic scarp scales. “International Commission on Large Dams” recommendations (1989) have been
followed.
The main results of the seismic hazard diagnosis for the Gibe III dam are the following:
• Seven seismogenic faults have been considered and characterized in terms of location, extension,
associated seismicity, and kinematics;
• The magnitudes of the design earthquakes recommended by ICOLD (1989) [Maximum Credible
Earthquake (MCE); Maximum Design Earthquake (950 yr-MDE); Operating Basic Earthquake (145
yr- OBE)] have been derived for each of these critical faults. Moreover, curves of magnitude versus
return periods have been produced for each seismognenic fault;
• Horizontal PGAs associated to design earthquakes have been derived for each critical faults at the
Gibe III dam site;
• For each design earthquake, the relative significant duration (time interval between 5 and 95% of
total areal intensity) was estimated using the equations of Kempton & Stewart (2006).
The detailed seismic assessment has been illustrated in the relevant report [300 GEN R BR 001 A - Seismic
Hazard Assessment, February 2007]. Some additional observations are recalled here below. The occurrence
in 1973 of an earthquake centred in the vicinity of the Weyto horst (about 50 km south of the study area) has
been mentioned. Since the turn of the century, nine earthquakes with a magnitude greater than 4, have been
recorded in the area. Three more have occurred in northern Kenya just south of the area, one to the north and
three close to the northeast, the latter being associated with the Main Ethiopian Rift. Three of these have
been recorded since 1950. Most were in the immediate proximity of the Chew Bahir rift and its northern
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