Although the knowledge and technology of seismic analysis and seismic risk assessment tools have rapidly advanced in the past several decades, current seismic design codes and damage estimation methods ignore the effect of successive earthquakes on structures. In light of recent strong seismic events, mainshock-damaged structures are shown to be more vulnerable to severe damage and collapse during subsequent events. The increase in vulnerability during aftershocks results in the likelihood of increased damage and loss-of-life and property. After a major earthquake, structural engineers must assess whether mainshock-damaged buildings can be re-occupied or not, with due consideration to the threat of aftershocks. The outcome of this post-earthquake inspection is utilized to quantifiably judge the current status of structures (so-called building tagging). This tagging criterion is closely related to the evaluation of the residual capacity of damaged buildings as well as the computation of the probability of being in a damage state after an aftershock (aftershock fragility). The increased vulnerability estimation associated with the additional damage plays a significant role in assessing potential losses to facilitate crucial decision making such as emergency response mobilization, inspection priority, recovery strategy, and re-occupancy decision. The main objective of this research is to develop a probabilistic framework for accounting for these increased vulnerabilities in terms of the extent of damage associated with mainshock ground motions. Aftershock fragility curves are developed accounting for both the uncertainty from the seismic hazard and the uncertainty from the structural capacity. This proposed approach also allows for the inherent variability, such as modeling characteristics associated with the design codes, present in non-ductile and ductile reinforced concrete frames found in California.