How does the Mercalli scale and the Richter scale measure the strength of an earthquake?
Both are scales used to measure earthquakes. However, they both go about it in different ways.
Mercalli intensity scale
This scale estimates the effects of an earthquake on the humans, nature, buildings, and the Earth’s surface from I (nothing felt) to XII (complete demolition). The values are dependent on distance from the earthquake, and the data is gathered from people who have experienced the earthquake. After, an intensity value is given the location of the witness. Remember, the Mercalli scale is dependent on structural damage–it is not defined by quantifiable measures like shake frequency, peak acceleration, or shake amplitude.
Richter magnitude scale
The Richter scale measures the amount of seismic energy released by an earthquake on a scale from 1 to 10. This scale is expressed logarithmically–an earthquake that measures 7.0 is 10 times larger than an earthquake that measures 6.0 and 100 times larger than an earthquake that measures 5.0. It is a logarithm of the earthquake’s amplitude of waves recorded by seismographs.
In summary, the Richter scale measures earthquakes quantitatively (measurements) and the Mercalli scale measures earthquakes qualitatively (experiences). Here is how the two scales match up and correspond to each other:
| Richter | Mercalli |
| 1.0-3.0 | I |
| 3.0-3.9 | II-III |
| 4.0-4.9 | IV-V |
| 5.0-5.9 | VI-VII |
| 6.0-6.9 | VII-IX |
| 7.0 and higher | VIII and higher |
The Richter Scale is logarithmic. That means a 2.0 on the scale is ten times more powerful than a 1.0 on the scale and a 3.0 would be a hundred times more powerful. The main problem with the scale is the dispersion of the shock waves as it travels to the recording sites (multiple sites are needed to pinpoint the epicenter) the material the shock wave passes through may impede or intensify the vibrations. None of my geology text books say much about the Mercalli Scale, but they are 15 years old. It is used to measure the damage on the surface. This can be misleading because the damage can relatively minor on the surface but quite extensive 100 m down.
