Measuring earthquake magnitude and intensity
Earthquake magnitude (M)
A numerical quantity to characterise earthquakes in terms of the total energy released. Moderate and strong magnitude earthquakes are in a range of M5.0 to M6.9; major magnitude earthquakes reach M7 and higher. Various earthquake magnitude scales exist, including local (or Richter) magnitude:
Richter scale
The Richter magnitude scale is a mathematical device to comparethe size of earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude and wavelength (A/T) recorded by seismographs. On the Richter scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value. (source: USGS National Earthquake Information Center, 1998)
Earthquake intensity
Today, the Modified Mercalli scale is commonly used to rank earthquake intensity according to the kind and amount of damage produced:
Modified Mercalli Intensity scale (MMI)
A numerical index describing the physical effects of an earthquake on man and man-built structures. MMI classes range from I to XII. The intensity at a given point depends not only on the strength of the earthquake (magnitude) but also on the distance from the earthquake to the epicentre and the local geology at that point. This is measured with the Mercalli Scale.
Mercalli intensity scale and witness observations:
Intensity
|
Witness observations |
|
|
|
| I |
Felt by very few people; barely noticeable
|
| II |
Felt by a few people, especially on upper floors.
|
| III |
Noticeable indoors, especially on upper floors, but may not be recognised as an earthquake. Hanging objects swing.
|
| IV |
Felt by many indoors, by few outdoors. May give the impression of a heavy truck passing by.
|
| V |
Felt by almost everyone, some people awakened. Small objects move. Trees and poles may shake.
|
| VI |
Felt by everyone. Difficult to stand. Some heavy items of furniture move, plaster falls. Slight damage to chimneys possible.
|
| VII |
Slight to moderate damage in well-built, ordinary structures. Considerable damage to poorly built structures. Some walls may fall.
|
| VIII |
Little damage in specially built structures. Considerable damage to ordinary buildings, severe damage to poorly built structures. Some walls collapse.
|
| IX |
Considerable damage to specially built structures, buildings shifted off foundations. Noticeable cracks in ground. Wholesale destruction. Landslides.
|
| X |
Most masonry and frame structures and their foundations destroyed. Ground badly cracked. Landslides. Wholesale destruction.
|
| XI |
Total damage. Few, if any, structures standing. Bridges destroyed. Wide cracks in ground. Waves seen on ground.
|
| XII |
Total damage. Waves seen on ground. Objects thrown up into air.
|
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