a change in the center of gravity, which would require an external force.

Of the fundamental toroidal modes, ₀T₁ represents changes in Earth's rotation rate; although this occurs, it is much too slow to be useful in seismology. The mode ₀T₂ describes a twisting of the northern and southern hemispheres relative to each other; it has a period of about 44 minutes.

The first observations of free oscillations of the Earth were done during the great 1960 earthquake in Chile.

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