WASHINGTON (AFP) - The "moment magnitude" scale signified by the Mw sign, now a quarter-century old, was conceived to remedy shortcomings in the Richter scale, considered ineffective in measuring extremely large quakes.
It was used by the US Geological Survey to measure Wednesday's devastating earthquake that rocked Peru, leaving at least 500 dead.
The earthquake reached an 8.0 magnitude, an exceptionally high number.
Conceived to avoid large differences with the Richter scale, the moment magnitude scale measures energy released by an earthquake.
It is an open logarithmic scale. Each time a new grade is attained, the amount of released energy is multiplied by 32.
To calculate the magnitude of an earthquake according to that method, the following are taken into account: a coefficient of the elasticity of rock (which is a constant value), the area of the rupture and the average movement of the plate.
According to the USGS, "moment is a physical quantity proportional to the slip on the fault times the area of the fault surface that slips; it is related to the total energy released" in the earthquake.
Roughly, an earthquake with a 6 magnitude would create a rift 10 kilometres (6.25 miles) long and would cause the plate to move some 10 centimetres (4 inches).
For a 7 magnitude, those numbers would be on the order of 100 kilometres and one metre, while an 8 magnitude would cause a rupture of several hundred kilometres and movement of more than 10 metres.
The moment magnitude and Richter scales measure an earthquake's magnitude, as opposed to those such as the Mercalli scale, which measure intensity, or effects observed near the epicentre.