Last week on March 11, 2011 a powerful earthquake occurred with an epicenter 80 miles off the coast of Japan near the city of Sendai, which has a population of about one million people. The Clear Science staff has heard the magnitude of the Sendai earthquake reported from 8.8 - 9.0 on the moment magnitude scale (MMS). The MMS is similar to the Richter scale, a name which is still sometimes used colloquially. (And why not? People know what it means!)
The MMS is a logarithmic scale, meaning that each number higher is actually ten times more powerful in magnitude. That’s why a big truck going by your house might be a 3-4 on the scale, but a huge earthquake that results in massive loss of life will be an 8 or 9. The largest earthquake ever recorded on Earth was the 1960 Valdivia earthquake in Chile, which was a 9.5. If you run through the logarithm math, this is 3.2 times larger than the Sendai earthquake that just happened.
The measurement scale for sound pressure, decibels, is also a logarithmic scale, in which small changes in number signal huge changes in loudness. (Decibels or dB are actually a logarithm multiplied times 20, so every 20 is 10x higher.)