The outer core of the earth, being in the plasma state with high temperature, high pressure, high density, and immersed in a strong toroidal magnetic field, can support varieties of plasma waves. Under influene of earth rotation, MHD wave propagates alone field lines in two modes; eastward inertal mode and westward magnetic mode, both are characterized dy high dispersion. The magnetic mode is related with the secularvariation and westward shift of the main magnetic field. The inertial mode has periods of the order of days, its magnetic effects would be shielded from the earth's surface dy conductive mantle. However, the mechanical couplins between the core and the lower mantle would be possidle, hence a periodic mechanic process would de produced by MHD wave in the earth's core.