FYI: Who was Richter and what is so special about his scale?

Scientists use two fundamentally different types of scale to describe earthquakes. The original energy of an earthquake is measured on a magnitude scale - regularly used by the media - while the intensity of shaking occurring at any point on the Earth's surface is measured on an intensity scale.

While the Richter is the most widely recognised magnitude scale, modern seismologists prefer to use the moment magnitude scale (MMS), which was introduced in 1979 by Japanese scientist Hiroo Kanamori and Thomas Hanks of the United States and gives a more reliable estimate of size for very big earthquakes. The MMS is calibrated so that it is almost identical to the Richter scale for values below 7.0.

Not to be confused with the other Richter scale, used to tune harmonicas, Charles Richter's assigns a single number to quantify the amount of seismic energy released by an earthquake. Seeking a way to differentiate the severity of earthquakes, the American seismologist and physicist recognised that the seismic waves radiated by quakes can provide good estimates, measured by the ground motion at a given distance, of their magnitude. In 1935, and in partnership with German-born Beno Gutenberg, both of the California Institute of Technology, he developed a scale from these measurements to use in a particular study in California, a quake-prone state in the US.

Basically, an earthquake that measures 5.0 on the Richter scale results in 10 times more ground shaking, and releases 31.6 times the amount of energy, than one that measures 4.0. Richter chose a magnitude 0 event to be an earthquake that would show a maximum combined horizontal displacement of one micrometre 100km from the epicentre. This choice was intended to avoid negative magnitudes but, as his scale has no upper or lower limits, sensitive modern seismographs routinely record quakes with negative magnitudes.

Values given by different seismological observatories for the same earthquake may vary because seismologists use several methods to estimate the magnitude of an event. Seismologists often revise estimates as they obtain and analyse data, as has happened with the May 12 Sichuan quake, which has been upgraded from 7.8 to 8.0.

The intensity, and thus ground effects, of an earthquake depends not only on the magnitude but also on the distance to the epicentre, the depth of the quake's focus beneath the epicentre and geological conditions - certain terrains can amplify seismic signals - which explains why the Sichuan event was so much more destructive than last year's Antofagasta quake, in Chile, which measured 7.7 but is thought to have claimed only two lives.