As there is no evidence that cracks are created directly in modes II and III, shear cracks probably occur only along a weakness plane, as in a preexisting fault. Mode I fracture therefore may be an important factor in crack formation during shallow earthquakes. A three-dimensional shear dynamic rupture process was simulated on the assumption that shear slip occurs only in a preexisting fault and the possibility of introducing new internal cracks that propagate under tensile stress as a consequence of the dynamic process of shear slip propagation. The discrete element method (DEM) was used to solve this problem because it can introduce internal tensile cracks. The simple slip-weakening model was used as the friction law on the preexisting fault for shear rupture propagation. For new tensile cracks, fracture follows classical Griffith theory when the critical value for tensile fracture surface energy is reached. The proposed model was used to simulate the rupture process of a strike-slip shallow fault. Results show that the generation of new cracks is affected by rupture directivity in terms of the hypocenter and asperity location as well as by fault geometry with respect to the free surface. Cracks develop a flower-like structure that surrounds the preexisting fault. When the asperity is located at less than a certain depth, the flower-like structure that originates from the top of the fault reaches the free surface. We consider that this is the mechanism for forming the flower structure near surface during a strike-slip shallow earthquake.