The harmonics at which any body, e.g. the Earth, tends to vibrate most freely, i.e. resonates. There are two fundamental types: torsional (vibration with motions perpendicular to the Earth's radius); and spheroidal (vibrations that are both radial and tangential to the Earth's surface). The study of such resonances, e.g. those induced by major earthquakes, provides information on the internal nature of the Earth. A major earthquake can make the entire globe vibrate or ring like a bell, and some earthquakes have been so large that sensitive seismometers have continued to record the oscillations for weeks after the event. The decay of the vibrations gives valuable information about the elastic layering of the Earth, and especially of the low velocity zone. Moonquakes produce similar phenomena.
Subjects: Earth Sciences and Geography.