A later phase of large-amplitude is observed at a station in Hokkaido from several events of the December 1991 off-Urup earthquake swarm in the Kurile Islands region. The phase is characterized by a narrow frequency-band with a dominant period of about 15 s and retrograde particle motion in a vertical plane with a strike very different from the great-circle direction between the station and the epicenter. The phase is observed from only the earthquakes that are located at shallow depths beneath the inner wall of the Kurile trench where sea depth is greater than 4 km. These characteristics indicate that the phase is a packet of Rayleigh waves that arrive at the station from an azimuth considerably different from the great-circle direction. Rayleigh waves with periods around 15--20 s are well excited by the shallow earthquakes beneath the inner wall of the trench. This is supported by the dominance of a spectral peak at 15--20 s in the spectra of the direct Rayleigh-waves and the later phase. Rayleigh waves that propagate in an oceanic region are strongly influenced by the thickness of the water layer at periods shorter than about 25 s. The group velocity decreases by 80% at 15 s with the increase of the sea depth from 4 km to 6 km. The sea-trench topography therefore forms a strong low-velocity zone. The observed later phase can be interpreted as a Rayleigh-wave packet guided by this velocity complexity. ¿ American Geophysical Union 1992