Faulting of strike-slip type in a vertically inhomogeneous medium is analyzed using antiplane models. It is shown that asperities boundaries where slip exhibits significant change may be associated with regions in which a rapid transition occurs from high to low rigidity and/or from high to low pore pressure. These boundaries are connected with stress concentrations within the ruptured regions where tectonic stress increases after faulting. Release of these stresses (aftershocks, interseismic earthquakes, aseismic faulting) and tectonic loading makes asperities boundaries a probable initiation site of a major earthquake during the following crustal deformation cycle. The present results also show that low rigidity layers should be deprived of driving stress during consecutive earthquake cycles, which explains the suppression of major earthquake ruptures and low level of seismicity that is generally observed in sediments.