Aftershocks of the 17 August 1999, Mw = 7.4, Kocaeli earthquake were used to investigate the large-amplitude SmS arrivals in the Sea of Marmara. We produced the event record sections from the transverse components of S and SmS arrivals at a distance of 10 to 200 km for the data recorded towards the east and west in the Sea of Marmara. For the data recorded towards the west, we observed that the amplitude of SmS phases enhances the ground motion by a factor of 2 to 3. The dipping Moho and/or anisotropy in the upper mantle or deep convex structure or deep lens are possible sources for the large-amplitude SmS phases. Since the study area has a potential of creating damaging earthquakes in the future, the effect of large-amplitude SmS phases on the ground motion should be considered in the studies of the seismic hazard of the region.