  The earthquake in December 2004 caused free oscillations of the Earth.
Vibrates the earth in the football mode 0S2, splits the natural frequency due
to rotation into five closely adjacent individual components. The sum of these
spectral components in the frequency band near 309 {\mu}Hz produces a beat that
gives the overall amplitude envelope a characteristic, regular pattern. From
the measured envelope the parameters frequency, amplitude, phase and damping of
generating sinusoids can be reconstructed. Since the method is extremely
sensitive to changes in frequency and phase, these quantities can be determined
precisely. The results depend on the geographical location of the site. Further
results are the half-life of the amplitude decrease and the resonator Q. It is
shown that the interaction of the five individual frequencies can be
interpreted as amplitude modulation, which requires a nonlinear process in the
Earth's interior.
