the scale is applied after the earthquake by conducting surveys of people's response to the intensity of ground shaking and destruction. thus, a given earthquake will have zones of different intensity all surrounding a zone of maximum intensity.
show in the table below.
note that correspondence between maximum intensity and richter scale magnitude only applies in the area around the epicenter. thus, a given earthquake will have zones of different intensity all surrounding a zone of maximum intensity.
the mercalli scale is very useful in examining the effects of an earthquake over a large area, because it will is responsive not only to the size of the earthquake as measured by the richter scale for areas near the epicenter, but will also show the effects of the efficiency that seismic waves are transmitted through different types of material near the earth's surface.
useful for determining the size of earthquakes that occurred before the modern seismographic network was available (before there were seismographic stations, it was not possible to assign a richter magnitude)