ABSTRACT
Earthquakes are localised hiccups in the convection of the mantle and occur where the material is too cool to deform smoothly and breaks discontinuously. Its temperature is below about half of the melting temperature on the absolute (Kelvin) temperature scale. It includes the crust but is restricted to a small fraction of the mantle, the lithosphere, roughly the uppermost 100 km, and the subduction zones, where lithospheric slabs descend into the mantle, retaining sufficient coolness to depths up to 700 km. Seismic energy is derived from the convective motion and is only a fraction of the convective energy of the whole mantle, itemised in Section 7.4, but represents a concentration of it. Earthquake energies are generally discussed in terms of magnitudes, M, of which there are some minor variants, mostly based on original work on surface waves from Californian earthquakes by C. F. Richter. The Richter surface wave magnitude, M S, is the logarithm of the ratio of the amplitude of a seismic wave (in microns) to its period (in seconds), normalised to observations at 100 km from the source and adjusted for source depth. Since this ratio is a measure of the elastic strain energy density of a wave, there is a direct relationship between M S and the energy, E, of an earthquake, which, with allowance for duration of the wave train, is
