The Mercalli Scale
The Mercalli Scale, the older of the two, measures the intensity of the earthquake, i.e. the impact of a quake on people and their property.
This speaks to the observed effects of an earthquake over a limited geographical area. Intensity scales assign whole numbers usually from 1 to 12 to describe these observed levels of shaking. An intensity of 1 means the earthquake was not felt, while 12 means absolute and total destruction.
In Jamaica, we formerly used the Modified Mercalli Scale (1956 version). Now we use the European Macroseismic Scale (1992), which has been developed and tested over a period of years by a working group of the European Seismological Commission.
The EMS makes the imprecise and subjective nature of assigning intensities more robust and straightforward with regard to earthquake effects on humans, objects and buildings.
The Richter Scale
The Richter Scale measures the magnitude or amount of energy released by the earthquake. This is a measure of size for earthquakes based on recordings of ground motions by instruments.
Charles Richter developed the first magnitude scale in 1935. He used the logarithmic scale (which scales numbers by a factor of 10) to accommodate the wide range of ground motions, which earthquakes can cause, and this has persisted to the present.
Scales today are based on various aspects of the seismograms as told by the following names: body-wave magnitude, duration magnitude, and moment magnitude. Jamaica uses the latter two scales for magnitude. The moment magnitude is the truest indication of the size of an earthquake because it is based on the amount of movement on the fault.