In the diagram below the dotted line represents the "standard" Richter earthquake. This standard earthquake is 100 km away and produces 1 mm of amplitude on the seismogram. It is assigned a magnitude of 3. Other earthquakes can then be referenced to this standard.

Note that a 100 km-away earthquake of magnitude 4 would produce 10 mm of amplitude and a magnitude 5 would produce 100 mm of amplitude: 1, 10 and 100 are all powers of 10 and this is why the Richter Scale is said to be "exponential." A change of one unit in magnitude (say from 4 to 5) increases the maximum amplitude by a factor of 10.