In this study, the seismic impact between two adjacent structures with different lateral stiffness is investigated through a computational model. The method used for the impact analysis in this study is based on the Hertz contact law combined with a non-linear damper activated during the impact to allow consideration of energy dissipation during the impact period. The selected analysis method is explicit time domain due to transient and non-linear nature of impact. The analysis explicitly includes the soil domain and structures, integrated in a single Finite Element (FE) model. Response history analyses are performed for various ground motions, and results are extracted in terms of impact force and acceleration response at various locations within the structures. The effect of uncertainties in local stiffness of concrete components in the impact zone are investigated to demonstrate the sensitivity of the results to variability of the impact parameters. As the impact force is found to be high enough to potentially cause structural damage, the seismic fragility of the external shear wall most susceptible to damage in the impact interface is also evaluated. Even though the mass of the two buildings in this study are on the same order, the seismic impact is shown to cause negligible changes in the In-Structure Response Spectra (ISRS) of the stiffer structure while significantly increasing the ISRS amplitudes of the more flexible structure.