Abstract: The strike directions of earthquake faults of many strong earthquakes occurred recently in North China show that there exists a stress field, the direction of its principal stress is nearly the same everywhere. Based on the Coulomb-Navier strength theory, the present author proposes in this paper the concept of fault with critical strike and discusses the physical mechanism forming the blocked segment of fault. Some characteristic features of the offset of a fault and the stress field around the block-segment as worked out by the same author previously are referred.By combining all such results, a method for defining dangerous areas of strong earthquakes is now available. In addition, a method for predicting the magnitudes of earthquakes is also proposed.