Monday, April 18, 2011

Tohoku quake and free oscillations

Large earthquake cause the Earth to "ring like a bell," the phenomenon of free oscillations.

There are nice animations of the different toroidal and spheroidal modes of free oscillations at this web page from Lucien Saviot of the CNRS.  Note the periods of the oscillations.

The image below shows the response of the superconducting gravimeter at Metsähovi, Finland.  The gravimeter is also a long period seismometer. Time runs from the day of the quake on the left to five days later on the right.  Hotter colors are higher amplitudes.  The scale on the left gives the frequency of oscillation, where f=0.0006 cycles/second corresponds to a period of oscillation of 28 minutes.

Another reprsentation of some of the free oscillation data in the lower part of the frequency spectrum is shown below, from the same source.
 The lowest mode is spheroidal mode 0S2 (54 minutes). All modes decay after a few days, except the lowest radial mode (0S0) which has an expected lifetime of 3-4 months. The radial oscillation mode changes radius of the Earth with the period of 20 minutes. The observed amplitude at Metsähovi is 0.06 mm on 17.3.2011.

Free oscillations of the Earth which had been theoretically predicted were first observed after the 1960 Chile quake, the largest earthquake ever recorded.

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