The Richter scale is the one most commonly referred to. For mathematical reasons, Richter is accurate only up to about 6.5. For larger quakes, scientists use the newer moment magnitude scale.
There are several ways to measure an earthquake, but the most common is magnitude. Scientists no longer use the original Richter scale, but an updated version. Earthquakes should be referred to as “magnitude X” rather than “an X on the Richter scale.” A magnitude 6.0 earthquake releases 32 times more energy than a magnitude 5.0 and nearly 1,000 times more energy than a 4.0. But that doesn't mean the ground shakes a thousand times harder in a 6.0 than a 4.0, because the energy is released over a much larger area.
How much power does an earthquake pack? A magnitude 6.0 quake releases approximately as much energy as 6,270 tons of TNT, an M 7.0 199,000 tons, an M 8.0 6.27 million tons and a M 9.0 99 million tons. Of course, all that energy is not focused in one particular spot, but spreads out in waves.
 “Quake Basics—Measurements.” Faultine: Seismic Science at the Epicenter. Retrieved August 30, 2011.
 “Earthquakes – A Whole Lotta Shakin’ Goin’ On.” State of California – Department of Conservation. Retrieved September 29, 2011.