Structure and Composition of the Earth

Structure of the Earth as revealed by geophysical data. Seismic methods constitute the main source of our knowledge about the constitution of the deep-seated parts of our planet. The region in which stresses accumulate before an earthquake and from which seismic waves emanate at the time of seismic impact, is known as its epicentre. Seismic waves are low-frequency elastic waves in the solid body of the planet. The dimensions of the epicentre may be of the order of a few tens of kilometres. Barring extremely forceful shocks, the main part of an earthquake’s energy is released in about one second. The amount of energy released under a very severe earthquake is of the order of 10 18 J.