An outstanding slow wave train similar to T wave in appearance has been observed on records of an ocean bottom seismometer array off Boso, central Japan. These slow wave trains are not observed on land, are usually spindle-shaped and dominant in vertical component, and have apparent velocities of ~1.3 km/s along the array. In order to understand the observations, we calculate synthetic seismograms by a finite difference method for horizontally stratified layers of seawater, sediment, and basement, which show a well-defined wave train consistent with the observations only if the source is placed above the basement layer. This result and normal mode calculations indicate that the slow wave train comprises essentially the sediment-trapped shear waves coupled with ocean acoustic waves. The observed events of slow wave trains occur near the boundary between the outer are high and the forearc basin, where vertical differential motion has been suggested to occur. We suggest that these small events represent the shallowest crustal activity unique to the triple junction off Boso. ¿ 2001 American Geophysical Union