Officially it's called the moment magnitude scale. It's a logarithmic scale, meaning each number is 10 times as strong as the one before it. So a 5.0 earthquake is ten times stronger than a 4.0 ...

These scales account for the distance between the earthquake and the recording seismometer so that the calculated magnitude should be about the same no matter where it is measured. Another scale is ...

There are millions of 2.5 (or less) magnitude earthquakes per year. Here is the magnitude scale: 2.5 or less: typically not felt, but can be recorded. 2.5 to 5.4: often felt, but only causes minor ...