[1] The importance of the damage effect on dynamic earthquake slip behavior is clarified using the damage tensor variable. Dynamic mode II faults that are embedded in damage-inducing media are assumed to relate to damage effects. This study confirms that the rupture velocity for spontaneous fault tip growth in the damage medium is reduced, being slightly smaller than rupture velocities observed for classical elastic material. Inelastic energy loss resulting in damage is relatively small but nonnegligible to the accumulated fracture energy. Two framework parameters employed in this study determine microcrack orientations and modes of microcracks and provide information on deviations in microcrack distribution between isotropic and nonisotropic ones. Spatial microcrack distribution patterns in isotropic and homogeneous systems are strongly dependent on a parameter related to the damage creation criterion. When considering secondary faulting, two apparently different behaviors, pulverization and branch development, are understood in a single framework, which gives a new insight into fault zone formation. The present results show that damage effect must be treated as a tensor variable rather than a scalar one as employed in many previous studies.Citation: Suzuki, T. (2012), Understanding of dynamic earthquake slip behavior using damage as a tensor variable: Microcrack distribution, orientation, and mode and secondary faulting,