Free oscillations of the Earth This phenomenon is a result of interference between two surface waves traveling in opposite directions. In fact it is a surface standing wave. Interference of Rayleigh waves results in spheroidal oscillation S while interference of Love waves gives toroidal oscillation T. The modes of oscillations are specified by three numbers, e.g. nSlm, where l is the angular order number (or spherical harmonic degree, see Spherical harmonics for more details). The number m is the azimuthal order number. It may take on 2l+1 values from -l to +l. The number n is the radial order number. It means the wave with n zero crossings in radius. The fundamental spheroidal mode 0S2 has a period of about 54 minutes. The period of 0S3 is 36 minutes, and 0S4 is 26 minutes. Two modes cannot naturally exist are the 0S0 and 0T1. The spheroidal mode 0S0 doesnt exist because it requires a change in the center of gravity, which cant happen. The toroidal mode 0T1 doesnt exist because it requires the entire sphere to twist, which violates the conservation of angular momentum. The fundamental toroidal mode 0T2 has a period of about 44 minutes [1]. For spherically symmetric Earth the period for given n and l does not depend on m. The first observations of free oscillations of the Earth were done during the great 1960 earthquake in Chile. Presently periods of thousands modes are known. These data are used for determining some large scale structures of the Earth interior.
Posted on: Tue, 08 Apr 2014 01:29:18 +0000
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