Stress and Deflection in the Lithosphere near Lake Kariba?I

Abstract

Summary Calculations of incremental stress and deflection in the crust have been made as part of a study of load-induced seismic activity at Lake Kariba. A general method is described for computing any desired stress components anywhere in an elastic half-space near a load of any shape on its surface. The program also computes the elastic depression and the gravitational energy converted to stored elastic strain energy. The static incremental stress and depression in the lithosphere near Lake Kariba are discussed with the aid of vertical sections and maps. The maximum shear stress under the deepest part of the lake rises to 2.12 bars, and the downward normal stress to 6.68 bars. The maximum depression in the Sanyati Basin is 23.5 cm. Computed depression differences along the Makuti-Kariba road are shown to be in excellent agreement with results of repeated precise levelling. The deflection is therefore mainly elastic and Young's modulus is near 0.85 megabars for the lithosphere in the area. Some evidence is available which suggests vertical movements of blocks on fault planes, no doubt associated with the earthquakes. Extended relevelling should give results of great scientific and engineering value. Stress differences produced by the lake in the upper mantle are too small to produce flow there, so that inelastic depression of the crust toward isostatic compensation of the load is not to be expected.