Megathrust earthquakes occur at subduction zones, accompanied by strong ground motion and tsunamis. Not only that, various types of slow earthquakes also occur on the megathrust faults, which are characterized by slower rupture process than that of regular earthquakes. To understand such complex slip behavior of the megathrust faults, we need to know physical properties of the megathrust faults. Currently I am tackling with receiver function analysis using ocean-bottom seismometer (OBS) data. Although receiver functions calculated with OBS data suffer from water- and sediment-related reverberations, they have potential to provide a unique constraint (i.e., shear modulus) on the megathrust properties.