Theoretical study of seismic waves including the first formulation of exact transient solution of seismic waves due to a dislocation in a semi-infinite medium (1973) and a new formulation of the extended Haskell matrices to simultaneously compute eigenfunctions of generalized Rayleigh-waves and Love-waves in an azimuthally anisotropic medium (1990).

Slow and silent earthquakes (slow slip events)

Study of slow and silent earthquakes, including discoveries of the large (Mw〜7.5) interplate slow faulting following the 1992 Sanriku-oki earthquake (Mw6.9) along the Japan trench (1995) and the 1989 Tokyo Bay silent earthquake (Mw5.9) (2000).　One consequence of the discoveries is the new concept of seismo-geodetic coupling, which is defined as the interplate coupling including aseismic faulting (2001).