To predict earthquakes, huge efforts have been devoted to monitoring earthquakes, crustal deformations and water level during past decades, however, has been found to be extremely difficult. A new approach in marine seismic studies on earthquake prediction proposes focusing on the nature of the subducting plate boundary. Some Ocean Drilling Program (ODP) drillings and seismic reflection studies show fluid flows and the existence of hydrous minerals in the decollement zone. Considering that the subducting plate might contain much water, a number of hydrous minerals might be stable down to 150-300 km, in particular, serpentines and lawsonite. In 1998, the authors carried out a seismic experiment at the Izu-Bonin trench using Ocean Bottom Seismometers (OBSs) and controlled sources. The 130 km long E-W line crosses the Torishima forearc seamount, one of the serpentine diapirs at the eastern terminus. The velocity structure obtained along the line shows a relatively high velocity at the top and a low velocity at the bottom of the serpentine diapir. The velocity of the mantle wedge is slower than that of normal mantle. The dip of the subducting slab is initially quite gentle and abruptly becomes steep around 100 km from the trench axis. Hydration of peridotite in the mantle wedge might occur close to the Izu-Bonin trench axis and serpentines seem to be raised upward to the ocean bottom.