Earth Normal Modes
Earth Normal Modes : Comprise the fundamental mode (with lowest frequency) and higher (frequency) modes (or "overtones") of the free oscillations of the Earth as being measurably excited by very strong earthquakes. They comprise spheroidal modes and toroidal modes. The longest period (about 54 minutes) has the spheroidal model 0 S 2, nicknamed "rugby" mode. The toroidal model 0 T 2 corresponds to a purely horizontal twisting motion between the northern and southern hemisphcer and has a period of about 44 min. See section 2.4 in Chapter 2 of this Manual and normal modes
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