Аннотация:The model of fluid-filled cracks migration from higher depth towards ground surface is developed. The source of such cracks is supposed to be situated in the vicinity of crust-mantle boundary so that average fluid pressure in the crack is close to lithostatic one. Nevertheless for the case of up-elongated cracks there is the fluid pressure difference between upper part of the crack and lower one. The crack motion results from the fact that the fluid pressure at the top of the crack is slightly more than lithostatic pressure in the surrounding rock and vise versa at the bottom. The thermofluctuational mechanism of rupture and healing of molecular bonds at the tip of crack is considered. Kinetic equation describing crack distribution over space and size is derived. Based on the solution of this equation we conclude that the crack size can increase due to both the crack interception and capture of small cracks by large one. It was found these process results in the increase of the velocity of the crack lifting as well as accumulation of large cracks in the upper crust. For certain parameters of rock, that can occur in the vicinity of the crust or mantle fault, the average crack size becomes equal to typical distance between the cracks followed by multiple cracks interception and possibly by the fracture of medium. The spot where this process can occur is situated at the distance 35-40 km far from the crust-mantle boundary. This effect allows us to advance a hypothesis that crust earthquake preparation process could be by-product of such effect. The estimations show the formation of focal zone could last for 1–300 years.