"Moment magnitude numbers scale such that that energy release increases by a factor of about 32 for each whole magnitude number. For example, magnitude 6 releases about 32 times as much energy as magnitude 5, magnitude 7 about 32 times as much as magnitude 6, and so on.
"This animation graphically compares the relative "sizes" of some 20th and 21st century earthquakes by their moment magnitudes (according to USGS/NEIC). Each circle's area represents its relative energy release, its color indicates its tsunami potential, and its label lists its moment magnitude, its location, and the year it happened."
Note, too, that the human cost may not seem proportional to the energy released...until you take into consideration population density near the epicenter/focus of the quake, local building methods, and earthquake preparedness.Japan's earthquake/tsunami preparedness is the best in the world, and you can be sure that were it not, the death toll of about 16,000 in 2011 would have been much higher.