IRIS Earth Science Week - Earthmodes

This Website was developed to accompany the 2007-2008 Earth Science Week (www.earthsciweek.org) Toolkit, specifically the lenticular titled “The Pulse of the Earth”.

Download a PDF of the lenticular here.

View animated GIF files of the “breathing” and “football” modes as illustrated on the lenticular here.

Earth's natural vibrational modes are strongly excited by "great" earthquakes, such as the M9.3 Sumatra earthquake of December 26, 2004. In addition to the “breathing” and “football” spheroidal modes that are shown on the lenticular, additional modes, such as the “pear-shape” mode, may be excited. Compare the “pear-shape” mode with the “breathing” and “football” modes in the QuickTime movies below. These movies show the approximate vertical motions (greatly exaggerated!) for the three spheroidal modes "breathing" (termed “0S0” by seismologists), “football” (0S2), and “pear-shape" (0S3) modes. The true amplitudes of these modes are on the order of 1 mm or less. The relative periods are correct (the actual periods for the Earth for 0S0, 0S2, and 0S3 are 20.5, 54, and 35.5 minutes, respectively).

Watch:

• "Breathing" 0S0 movie
• "Football" 0S2 movie
• "Pear-shape" 0S3 movie

The "spectrogram" below shows the approximate relative spectral energies for the six lowest frequency spheroidal modes from the Sumatra earthquake, plotted for a period of 20 days. The modes die out with time due to the planet's anelasticity (because elastic energy is gradually converted to heat).

The accompanying audio file demonstrates what these vibrations "sound" like, when they are sped up 200,000 times. The signals are 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 (it consists entirely of P-wave energy, which propagates with significantly less loss than modes that incorporate S-wave energy).

Listen:

• test.au

Thanks to Rick Aster at New Mexico Tech for providing the movies, spectrogram, audio file, and accompanying information.

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