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M=1 to 3: Recorded on local seismographs, but generally
not felt
M=3 to 4: Often felt, no damage
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M=5: Felt widely, slight damage near epicentre
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M=6: Damage to poorly constructed buildings and other
structures within 10's km
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M=7: "Major" earthquake, causes serious damage up
to ~100 km (recent Taiwan, Turkey, Kobe, Japan, and California earthquakes).
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The magnitude of an earthquake is a measure of the amount of
energy released. Each earthquake has a unique magnitude assigned to
it. This is based on the amplitude of seismic waves measured at a number of
seismograph sites, after being corrected for distance from the earthquake. Magnitude
estimates often change by up to 0.2 units, as additional data are included in
the estimate.
The Richter scale is logarithmic, that is an increase of 1 magnitude
unit represents a factor of ten times in amplitude. The seismic waves of a magnitude
6 earthquake are 10 times greater in amplitude than those of a magnitude 5 earthquake.
However, in terms of energy release, a magnitude 6 earthquake is about 31 times
greater than a magnitude 5.
The intensity of an earthquake varies greatly according
to distance from the earthquake, ground conditions, and other factors. The
Modified Mercalli Intensity Scale is used to describe earthquake effects.
For more information on earthquakes and earthquake hazards,
click here .
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