A better measure is the seismic moment , which measures the amount of energy released. Measured by determining the amount of slip and the area of the rupture surface. Ground acceleration is particularly important for structural failure. Another important measure of earthquake magnitude is the intensity .
Intensity is determined by doing surveys of what people experienced during an earthquake.
Talk of intensity leads us to a discussion of how different rock and soil types respond to earthquake waves. SO - we must look to the seismic waves. Consider the P-wave. Energy transported rather than mass, and danger of tsunamis is result of that energy. 1000 people killed by tsunamis from local M 7.5 earthquake.