There are two methods of measuring the scale of earthquakes. The Richter scale - named after US seismologist Charles Richter (1900-1985) - that measures the magnitude, or relative amount of energy released, of an earthquake at its epicenter. The Mercalli scale - named after Italian seismologist Giuseppe Mercalli (1850-1914) is a measurement of the intensity from the observed effects of an earthquake. This is a subjective scale in that the Mercalli Number will vary from place to place depending on the distance from the epicenter.
The Richter Scale is most useful to geologists in that it measures the amount of energy released by movements in the Earth's crust whilst the Mercalli Scale is more a measure of the effect on people and property.
For example a Richter Scale 3 event in an urban area could lead to intense damage and loss of life - rating a Mercalli Intensity of XI, while a Richter Scale 8 event under the Pacific Ocean may cause little discernible effect and be rated as Mercalli Scale I.
|II||Felt by a few persons especially on upper floors of buildings|
|III||Quite noticeable, standing motor cars may rock slightly|
|IV||Dishes, windows, doors disturbed; walls make a cracking sound|
|V||Felt by nearly everyone; windows broken|
|VI||Slight damage; felt by everyone; furniture moved|
|VII||Negligible damage in well built structures; considerable damage in poorly built or designed structures|
|VIII||Slight damage in well built structures; considerable damage in ordinary structures|
|IX||Damage considerable; buildings shifted off foundations|
|X||Masonry and frame structures destroyed; rails bent|
|XI||Bridges destroyed; few structures remain standing|
|XII||Total destruction; objects thrown into the air|