Abstract: The analysis on structures of several strong earthquake (Ms ≥ 8) epicentral regios shows j the position, where Internal Tensile Shear Fault (ITSF) intersects with Secondary Strike-slip Fault (SF) with larger displacemental amplitude than another one in a pull——apart basin which consists of both of them), is the place where strongest earthquake (Ms ≥ 8) will occur.Genesis of ITSF is discussed based on Coulomb Shear Failure Criteria and results of Clay Box Shear experiment.The Author suggests that the ITSF is Riedel Shear Fracture within a block which is under simple shearing.Finially, the formational geometric conditions of ITSF are discussed according to statistics, geological and geomorphological research results.