Abstract In this paper, we present archaeological evidence of seismic damage in the ancient Roman town of Ostia and we perform structural analysis on damaged buildings based on the application of the principles of arch mechanics, in order to provide an objective means to identify the seismogenic origin of the observed ruptures and collapses. We combine a review of literature reports on possible earthquake damages affecting the ancient structures with field investigations, aimed at selecting representative cases in which collapse modalities can be traced back and the seismic origin evidenced. Nine cases of failure affecting masonry structures are analysed, illustrating the collapse dynamics through the virtual arches model. Using this method we also reconstruct tentative collapse vectors for the analysed cases, highlighting iso-oriented, prevalent horizontal components, indicative of earthquake-induced ruptures.