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Seismic Scales


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.


The Richter Scale

MAGNITUDEENERGY RELEASED
11
230
31x103
43x104
51x106
63x107
71x109
83x1010
91x1012

The Mercalli Scale

INTENSITY VALUEDESCRIPTION
INot felt
IIFelt by a few persons especially on upper floors of buildings
IIIQuite noticeable, standing motor cars may rock slightly
IVDishes, windows, doors disturbed; walls make a cracking sound
VFelt by nearly everyone; windows broken
VISlight damage; felt by everyone; furniture moved
VIINegligible damage in well built structures; considerable damage in poorly built or designed structures
VIIISlight damage in well built structures; considerable damage in ordinary structures
IXDamage considerable; buildings shifted off foundations
XMasonry and frame structures destroyed; rails bent
XIBridges destroyed; few structures remain standing
XIITotal destruction; objects thrown into the air

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