What is the scale used by the United States Geological Survey to measure the size of earthquakes in terms of the energy released?

If you mention earthquake strength, many people immediately think of the Richter scale. However, it is the moment magnitude scale (MMS) that is used by seismologists to measure the size of earthquakes in terms of the energy released. The magnitude is based on the seismic 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 MMS was developed in the 1970s to succeed the 1930s-era Richter magnitude scale. Even though the formulae are different, the new scale retains a similar continuum of magnitude values to that defined by the older one.

Starting in January 2002 the MMS is officially the scale used by the United States Geological Survey to calculate and report magnitudes for all modern large earthquakes.

The other answers refer to alternative scales that have been proposed for measuring the strength of earthquakes.