What scale is used to measure the energy released by an earthquake?

The moment magnitude scale is used by seismologists to measure the amount of energy released by large earthquakes (those greater than magnitude 8.0).

For smaller earthquakes (those with magnitudes less than 7.0 and with epicentres less than 650 km from a seismometer station may be used) the method devised by Richter (the Richter magnitude scale) may be used to estimate the magnitude. The surface wave magnitude scale may be used for earthquakes with magnitudes up to 8.0 (devised by Richter and Gutenberg to extend the utility of the Richter scale.)

Richter magnitudes are generally easier to derive than moment magnitudes being based on direct seismometer measurements, whereas the moment magnitude is a more4 fundamental measurement of magnitude being based on the rock mass strength around the fault, the amplitude of fault movement and the cross sectional area of that portion of the fault that moved. However this is more difficult to measure.

As such it is common for initial reports to be in Richter magnitudes and more detailed letter magnitudes to be reported as moment magnitudes.