Time dependence and trigger mechanisms for the Kariba (Rhodesia) earthquakes

Correlations between time-dependent parameters of incremental stress and induced seismicity during the filling of Lake Kariba (Rhodesia/Zambia) support the hypothesis, originally advanced in 1970, that failure was at that time triggered by increment to the existing tectonic stress field. About three years after the mainshock of September, 1963, these correlations disappeared and it is suggested that the principal triggering mechanism has been increase of pressure in the water in pores and fractures of the rock, from mid-1966 through 1974. The Mohr failure criterion with Coulomb friction is used to show that both types of triggering are possible in a normal-fault régime. Triggering by incremental solid stress requires a friction coefficient < 0.577 in a normal-fault stress field. If the Okavango Delta, the Middle Zambezi Rift and the Luangwa Rift form a single member of the African Rift system, the only two flooded parts of this rift are now the main centres of seismic activity, namely the Okavango Delta and Lake Kariba.