of a body (eg, the Earth) at particular harmonic
s when excited by an event, such as an earthquake
There are two types: torsional and spheroidal
Torisional describes vibrations whose motions are perpendicular to the Earth's radius, while spheroidal describes vibrations that are both radial and tangential to the Earth's surface.
These oscillations provide information on the physical properties of the interior: for example, major earthquakes can 'ring the Earth like a bell', the oscillations continuing for weeks. In this way, the decay of the vibrations can tell geologists and seismologists about the difference between the layers within the Earth, and the specific properties of each.
Moonquakes have also been known to produce similar results and information.