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An Integrated Study of Reservoir-Induced Seismicity and Landsat Imagery at Lake Kariba, Africa

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An Integrated Study of Reservoir-Induced Seismicity and Landsat Imagery at Lake Kariba, Africa GREGORYB. PAVLIN CHARLESA. LANGSTON Department of Geosciences The Pennsylvania State University University Park, PA 16802 An Integrated Study of Reservoir-Induced Seismicity and Landsat Imagery at Lake Kariba, Africa A combined study of reservoir-induced earthquakes and Landsat Imagery yields information on geology and tectonics that either alone would not provide. I INTRODUCTION threat to life and property; (2) they offer a unique class of earthquakes which may be pre- ARTHQUAKES,depending upon their depth vented, controlled, and possibly predicted; (3) Eof origin and geographic location, are the re- they provide a class of earthquakes that can be sult of different types of physical mechanisms. easily monitored; and (4) the understanding of Perhaps the most interesting occurrences of the physics of these earthquakes would offer earthquakes are those linked, both directly and clues regarding the nature of other classes of indirectly, to the actions of men. Examples shallow earthquakes. ABSTRACT:The depth and seismic source parameters of the three largest reservoir-induced earthquakes associated with the impoundment of Lake Kariba, Africa, were determined using a formalism of the generalized inverse technique. The events, which exceeded magnitudes, mb, of 5.5, consisted of the foreshock and main event (0640 and 0901 GMT, 23 September 1963) and the principal aftershock (0703 GMT, 25 September 1963). Landsat imagery of the reservoir region was exploited as an independent source of infornation to de- termine the active fault location in conjunction with the teleseismic source parameters. The epicentral proximity and the comparable source parameters of the three events suggests a common normal fault with an approximate strike of S 9"W, a dip of 62", and rake of 266". The fault trunsects the reser- voir's deep Sanyati Basin. Body-wave modeling of the events infers source depths of less than 10 km. The photointerpretation of Lundsat imagery re- sulted in the mapping of over 400 lineaments, having trends of N 20°W to N 40°W, that trunsect the Mid-Zambezi Rift Valley region. These features seem to be correlated with joint fracture patterns and dykes injected with Precam- brian pegmatites and Jurassic dolerites. The lineaments and their intersec- tions with known faults may provide potential sites for future mineral ex- plorution. would include earthquakes triggered by subsur- This study is concerned with the nature of face nuclear explosions, subsurface injection of reservoir-induced seismicity (RIS) presumably fluids, and the construction of man-made reser- caused by the impoundment of Lake Kariba in voirs (Gupta and Rastogi, 1976; Healy et al., southern Africa. The geographical location of 1972; Dahlman and Israelson, 1977; Cough, Lake Kariba and the generally small to moderate 1978). Earthquakes generated by cultural activi- size of the earthquakes which occurred there are ties are of particular interest to the seismological factors which have limited previous seismologi- community because (1) they may pose a serious cal investigations. In the present study, we have PHOTOGRAMMETRICENGINEERING AND REMOTE SENSING, 0099-1112/83/49040513$02.25/0 Vol. 49, No. 4, April 1983, pp. 513-525. @ 1983 American Society of Photogrammetry PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1983 endeavored to use recently developed seismo- roles in the actual physical mechanisms of RIS logical techniques in conjunction with interpreta- (Gupta and Rastogi, 1976; Brace, 1972; Healy et tion of Landsat imagery to determine basic fault ul., 1972; Wahlstrom, 1974). parameters such as source depth, fault orienta- Some investigators have addressed the nature tion, and epicenter location. The rationale be- of the actual triggering mechanisms acting at hind this is eventually to build a catalog of ac- depth within the Earth's crust beneath the reser- curately determined source parameters from voir. Gough and Gough (1970b) initially identi- reservoir-induced earthquakes to be used in as- fied three mechanisms by which seismic activity sessing possible physical mechanisms relevant to could be potentially stimulated by the loading of the phenomemon of induced seismicity. an artificial lake; (1) the direct effect of the in- crease in shear stress at depth; (2) the indirect THEPROBLEM OF RESERVOIR-INDUCED effect of incremental stress in 'triggering' release SEISMICITY(RIS) of pre-existing tectonic stress; and (3) the effect Reservoirs have been constructed throughout of increased pore pressure acting upon dormant the world for many reasons including hydroelec- fault zones beneath the reservoir. Through their tric power, flood control, irrigation, commerce, study of the stresses induced by the impound- and recreation. Site selection of these reservoirs ment of Lake Kariba, Africa, Gough and Gough is principally a function of the structural, geolog- (1970a, 1970b) dismiss the mechanism attributed ical, and hydrological properties of the region to the direct load-induced increase in shear within which the reservoir is to be impounded. stress. Furthermore, RIS depends upon the type A small proportion of the hundreds of reservoirs of faults that may exist beneath the reservoir; constructed during the past century have been i.e., dip-slip normal, strike slip, or thrust faults. associated with a change in regional earthquake Kisslinger (1976) provides a comprehensive re- activity. In a few documented instances, RIS has view of the mechanisms for reservoir-induced achieved catastrophic proportions, which re- earthquake activity. More detailed analysis of the sulted in damage to the dam structure, extensive role of pore pressure as a possible triggering property damage, and loss of human life. An ex- mechanism has been reported by Bell and Nur treme example is the Koyna Reservoir of India, (1978). which is believed to have triggered the magni- Within the scope of determining the mecha- tude 6.4 earthquake which occurred there in nism of reservoir-induced earthquakes, seismo- 1967. Simpson (1976) and Gupta and Rastogi logical techniques have been utilized to con- (1976) provide excellent reviews of the RIS phe- strain the parameters that define the orientation nomenon that was first observed at Lake Mead, and depth of the fault plane upon which the res- Arizona, in the 1930's, by Carder (1945). ervoir-induced earthquakes occur. The fault Reservoirs associated with the RIS phenomena plane orientation is conventionally defined by have some common features. RIS is most com- geograhic location, depth, and the angular pa- monly associated with large reservoirs; large res- rameters of strike, 8, dip 6, and rake, A. By con- ervoirs being defined as having a volume in ex- straining the fault or dislocation parameters, in- cess of a million-acre feet (1.2 x lo9 m3) and cluding the depth of the given earthquake(s), impounded behind a dam in excess of 90 metres complemented by ancillary information of the (Simpson, 1976). Perhaps the key feature of RIS hydrological, geological, and tectonic environ- is that, typically, the earthquake frequency of the ment of the region, it may be possible to deduce region increases measurably following the filling the triggering mechanisms acting at depth be- of the lake. Epicenters are usually located within neath the reservoir. Achieving an understanding 25 km of the reservoir. By correlating the fre- of the triggering mechanism would offer hope for quency of the earthquakes' occurrence with the the eventual prevention, control, and possible fluctuations in reservoir water level, Gupta et al. prediction of the RIS phenomena. (1972) suggested that the following factors af- fected the frequency of seismicity: the rate of in- crease in water level; duration of loading; maxi- Lake Kariba, which borders Zambia and Zim- mum levels of water; and the time period for babwe-Rhodesia (16.8"S, 28.0°E), is the result of which the high water levels were maintained. a joint governmental action to develop a source Finally, some investigators have inferred that of hydroelectric power for the region. The reser- certain geological and hydrological conditions voir occupies the Mid-Zambezi Basin which is are necessary for such earthquake occurrences. considered to be a dormant southern extension of Case histories of the Koyna (India), Kariba (Zam- the East African Rift System. The reservoir is bia-Zimbabwe), and Kremasta (Greece) reser- formed by the impounded waters of the Zambezi voirs provide evidence that pore pressure River, as well as other rivers of the watershed re- changes, permeability, porosity, and pre-im- gion. The impoundment of the Kariba Reservoir poundment water table levels play important began in December 1958 through the operation RESERVOIR-INDUCED SEISMICITY FIG.1. Landsat imagery of the Lake Kariba region. Zambia borders the northern and western shores of the lake. Zimbabwe borders the lake to the south and east. Two images were concatenated to insure total coverage of the reservoir; both images are Band 6, collected on 30 September 1972. PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1983 of a dam that was constructed at the mouth of the for the period spanning June 1959 through Jan- Kariba Gorge, which is located about 420 km uarv 1979. downstream from Victoria Falls. Lake Kariba re- 1h September 1963, seismicity dramatically in- mains the world's largest artificial lake, extend- creased to new levels by the triggering of six ing over 250 km in length and about 30 km in earthquakes having magnitudes exceeding 5.0 on width. Having an area of approximately 6650 the Richter scale. Table 1 provides a synopsis of km2, the lake stores 1.53 x 10" m3 of water the major earthquakes that have been located when the water reaches an engineered maximum within the immediate vicinity of the reservoir. level of 484.6 m (1590 ft) above sea level (Gough The main event, of magnitude 6.0, caused dam- and Gough, 1970a; McConnell, 1978). Figure 1 age to the dam structure and some property dam- is a composite Landsat image, Band 6, of the res- age in nearby settlements; however, there were ervoir.
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