The strength of an earthquake is usually measured on
one of two scales, the Modified Mercalli Scale and the Richter Scale. The
Mercalli Scale is a rather arbitrary set of definitions based upon what
people in the area feel, and their observations of damage to buildings
The scale goes from 1 to 12, or using the descriptive titles of the levels, from Instrumental to Catastrophic.
|I||Instrumental||Detected only by seismographs|
|II||Feeble||Noticed only by sensitive people.|
|III||Slight||Resembling vibrations caused by heavy traffic.|
|IV||Moderate||Felt by people walking; rocking of free standing objects.|
|V||Rather strong||Sleepers awakened and bells ring.|
|VI||Strong||Trees sway, some damage from overturning and falling objects.|
|VII||Very strong||General alarm, cracking of walls.|
|VIII||Destructive||Chimneys fall and there is some damage to buildings.|
|IX||Ruinous||Ground begins to crack, houses begin to collapse and pipes break.|
|X||Disastrous||Ground badly cracked and many buildings are destroyed.There are some landslides.|
|XI||Very Disastrous||Few buildings remain standing; bridges and railways destroyed; water, gas,electricity and telephones out of action.|
|XII||Catastrophic||Total destruction; objects are thrown into the air,much heaving, shaking and distortion of the ground.|
Whilst this scale is fine if you happen to experience an earthquake in an inhabited area of a developed country, it is of no use whatsoever in the middle of a desert or in any other place without trees, houses and railways! Descriptions such as "Resembling vibrations caused by heavy traffic." depend very much upon the observer having felt heavy traffic in the past. Even then, what one person in a small town considers to be 'heavy' will most certainly differ from what a person living adjacent to a major urban road system would describe as 'heavy'.
Clearly this scale has advantages, but something else is required if we are to be able to compare the magnitude of earthquakes wherever they occur. The Intensity Scale differs from the Richter Magnitude Scale in that the effects of any one earthquake vary greatly from place to place, so there may be many Intensity values (e.g.: IV, VII) measured for the same earthquake. Each earthquake, on the other hand, should have only one Magnitude, although the various methods of calculating it may give slightly different values (e.g.: 4.5, 4.6).
The Richter Scale is designed to allow easier comparison of earthquake magnitudes, regardless of the location.
C.F.Richter was a geologist living and working in California, U.S.A, an area subjected to hundreds of 'quakes every year. He took the existing Mercalli scale and tried to add a 'scientific' scale based on accurate measurements that could be recorded by seismographs ( instruments used to measure vibration) regardless of their global location.
By measuring the speed, or acceleration, of the
ground when it suddenly moves, he devised a scale that reflects the
'magnitude' of the shock.
The Richter scale for earthquake measurements is logarithmic. This means that each whole number step represents a ten-fold increase in measured amplitude. Thus, a magnitude 7 earthquake is 10 times larger than a 6, 100 times larger than a magnitude 5 and 1000 times as large as a 4 magnitude.
This is an open ended scale since it is based on measurements not descriptions.
An earthquake detected only by very sensitive people registers as 3.5 on his scale, whilst the worst earthquake ever recorded reached 8.9 on the 'Richter Scale'.
When trying to understand the forces of an earthquake it can help to concentrate just upon the upward movements. Gravity is a force pulling things down towards the earth. This accelerates objects at 9.8 m/s/s. ( Not quite true because of air resistance etc, but that's another story). To make something, such as a tin can, jump up into the air requires a shock wave to hit it from underneath travelling faster than 9.8m/s/s. This roughly corresponds to 11 (Very disastrous) on the Mercalli Scale, and 8.1 or above on the Richter Scale. In everyday terms, for the tin to lift off the ground it must be hit by a force that is greater than that which you would experience if you drove your car into a solid wall at 35 khp (22 mph).
Mercalli Acceleration in cm/s/s Richter ScaleI 1 II 3.5 2.5 to III 4.2 5 IV 4.3 10 to V 4.8 25 VI 4.9 to 5.4 50 VII 5.5 to 6.1 100 VIII 6.2 250 to IX 6.9 500 X 7.0 to 7.3 750 XI 7.4 to 8.1 980 XII over 8.1
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