What measure of an earthquake's size does the Richter scale express?

Magnitude

The Richter scale measures magnitude by calculating the logarithm of the amplitude of waves recorded by seismographs. Because the scale is logarithmic, each whole number increase represents a tenfold increase in measured amplitude and roughly 31 times more energy release. This means a magnitude 7.0 earthquake is significantly more powerful than a 6.0, rather than just slightly larger.