An earthquake is the result of a sudden release of energy in the Earth’s crust that creates seismic waves. The seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time. Earthquakes are measured with a seismometer. The moment magnitude (or the related and mostly obsolete Richter magnitude) of an earthquake is conventionally reported, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causing serious damage over large areas.
The Richter scale was superseded by the moment magnitude scale, which is calibrated to give generally similar values for medium-sized earthquakes (magnitudes between 3 and 7). The magnitude is based on the moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of the area that slipped. The scale was developed in the 1970s to supersede the 1930s-era Richter magnitude scale.
Unlike the Richter scale, the moment magnitude scale reports a fundamental property of the earthquake derived from instrument data, rather than reporting instrument data which is not always comparable across earthquakes, and does not saturate in the high-magnitude range. Since the Moment Magnitude scale generally yields very similar results to the Richter scale, magnitudes of earthquakes reported in the mass media are usually reported without indicating which scale is being used.
Energy released by an earthquake closely correlates with destructive power and scales with the 3⁄2 power of its shaking amplitude. So a difference in magnitude of 1.0 is equivalent to a factor of 31.6 in the energy released, two magnitudes higher is equivalent to 1000 times more energy.
The original Richter magnitude scale assigns a single number to the amount of energy on a base-10 logarithmic scale. The value is the log of the combined horizontal amplitude (shaking amplitude) of the largest displacement from zero on a Wood–Anderson torsion seismometer. 8.0 on the Richter scale has a shaking amplitude ten times larger than a 7.0 earthquake.
An 8.9 magnitude earthquake, such as that which occurred this week in Japan, is the equivalent in energy of 336 million tonnes of TNT explosive at 1.4 exajoules (1.4 million, million, million Joules of energy).