  Tidal force plays an important role in the evolution of the planet-moon
system. The tidal force of a moon can excite seismic waves in the planet it is
orbiting. A tidal-seismic resonance is expected when a tidal force frequency
matches a free-oscillation frequency of the planet. Here we show that when the
moon is close to the planet, the tidal-seismic resonance can cause
large-amplitude seismic waves, which can change the shape of the planet and in
turn exert a negative torque on the moon to cause it to fall rapidly toward the
planet. We postulate that the tidal-seismic resonance may be an important
mechanism which can accelerate planet accretion process. On the other hand,
tidal-seismic resonance effect can also be used to interrogate planet interior
by long term tracking of the orbital change of the moon.
