There are many different instruments used to measure the size of an earthquake. Two of the most common instruments are the Richter scale and the mercalli intensity scale. They really measure 2 completely different things and use two types of rating scale.
The Richter scale measures magnitude, which is calculated by the height of the earthquakes waves measured by a seismograph. Magnitude is almost always measured on the Richter scale. The points 1-9 of the Richter scale corresponds to a factor of 10 increase in the amplitude of earthquakes waves.
Another scale used to describe the size of earthquakes shows the intensity of quakes based on eyewitness descriptions of the damage in the area. This scales used roman numerals from I~XII and is called the modified mercali scale.
As you can see the two scales are very different and if an earthquake happens near my home I would rather be informed of the damage done than just a simple number of intensity. This would help me prepare for what I would find as far as damage to roads, building and my city but the intensity will vary depending upon the place from which it was measured.(EE4)