The difference between the Richter and Mercalli scales, both intended to measure earthquake severity, is that the Richter scale is a mathematical scale based on the measurements of scientific instruments, while the Mercalli scale is based on witness observation and structural damage. The Richter scale is much more objective and scientific and is usable anywhere there is an earthquake. The Mercalli scale is useless outside of populated areas.
The Richter and Mercalli scales are both measures of earthquake severity, but the Richter scale is much more commonly used. The scientific nature of the Richter scale is based on the maximum amplitude of a wave recorded from an earthquake combined with the distance of the instrument from the epicenter of the quake. There are reasons, however, that the Mercalli scale is still used.
While the Mercalli scale does not actually say anything directly about the energy released in an earthquake, it speaks much more directly to the effects on human beings, which is the primary concern of governments dealing with the aftereffects. The Mercalli scale varies not just by the energy of the earthquake but by the nature of the surrounding landscape and the quality of the infrastructure. The same Richter magnitude earthquake can have a much greater impact in the Mercalli scale with older buildings or in areas prone to landslides.