Spectrogram Showing the Earth's Vibrational Modes

Earth's natural vibrational modes were strongly excited by the Sumatra-Andaman Islands earthquake (see: "At longest periods, the 2004 Sumatra-Andaman earthquake rang Earth like a bell, and the planet is still ringing", above). This "spectrogram" shows the approximate relative spectral energies for the six lowest frequency spheroidal (having both vertical and horizontal motion) from the earthquake, plotted for a period of 20 days.. The modes decay with time due to the planet's anelasticity The accompanying audio file demonstrates what these vibrations "sound" like, when they are sped up 200,000 times. For simplicity, the signals have zero relative phase lag and no rotational splitting, and are thus roughly representative of the vertical ground motion induced by an excitation at one of the planet's rotational poles. The gravest mode (0S2; the "football mode") is mapped to a sound frequency of just over 60 Hz by this process. The 0S0 "breathing" mode, which is mapped to about 160 Hz in the audio file, persists the longest because of its very low decay rate relative to the other modes.

Listen to the sound of the seismic waves (note: it is easier to hear with external speakers).

Figure by Richard Aster, New Mexico Institute of Mining and Technology