Large scale fluid motion due to the 2011 Tohoku Tsunami was investigated by decomposing the water surface elevation computed by the linear wave theory through spectrum analysis. Dominant periods of nearshore fluid motion are extracted from the water surface elevation at the depth of 10m. The power spectra and phases at the alongshore points are compared. From these results, cross-shore and alongshore long-period oscillations are described in Sendai Bay, which appear to be consistent with the large runup height especially at the center and the both ends of the Sendai Bay. It appears that the long-period motion with period of 80 minutes propagates southward alongshore as edge waves. Standing alongshore wave motion which is not consistent with the dispersion relationship of edge wave is also detected.
