Earthquake magnitude is a measure of the amount of wave energy radiated from the source. The magnitude of a particular earthquake is estimated from the amplitude of ground motion as recorded on a standard seismograph, taking into account the distance of the focus. On the Richter magnitude scale, the greatest known earthquakes have been of magnitude 8.9, while an earthquake of magnitude less than 2.5 is not likely to be perceived anywhere without the help of instruments. The greatest New Zealand earthquake in historical times was the Wellington earthquake of 1855, with a magnitude of about 8. The disastrous Hawke's Bay earthquake of 3 February 1931 had a magnitude of 7¾.

The precise numerical relationship between magnitude and energy is still uncertain. A recent version is

log10E = 11.8 + 1.5 M

where M is the magnitude and E is the energy in ergs. According to this equation, the energy varies by a factor of about 33 for each unit of magnitude. The energy released in a magnitude 7 earthquake is about 2 × 10 22 ergs. Man is now able, by detonating a thermonuclear explosion equivalent to some 50 million tons of TNT, to release an amount of energy comparable to that in a major earthquake.

Small earthquakes are very much more frequent than large ones; even so, the largest earthquakes account for most of the total energy released. The following table gives the approximate numbers of earthquakes, in successive magnitude ranges, occurring in New Zealand in an average year: