It is a base-10 logarithmic scale obtained by calculating the logarithm
of the combined horizontal amplitude (shaking amplitude) of the largest
displacement from zero on a particular type of seismometer (Wood–Anderson torsion).
So, for example, an earthquake that measures 5.0 on the Richter scale has a shaking
amplitude 10 times larger than one that measures 4.0.
||Frequency of occurrence
|Less than 2.0
||Micro earthquakes, not felt.
||About 8,000 per day
||Generally not felt, but recorded.
||About 1,000 per day
||Often felt, but rarely causes damage.
||49,000 per year (est.)
||Noticeable shaking of indoor items, rattling noises. Significant damage unlikely.
||6,200 per year (est.)
||Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well-designed buildings.
||800 per year
||Can be destructive in areas up to about 160 kilometres (100 mi) across in populated areas.
||120 per year
||Can cause serious damage over larger areas.
||18 per year
||Can cause serious damage in areas several hundred miles across.
||1 per year
||Devastating in areas several thousand miles across.
||1 per 20 years
||Never recorded; see below for equivalent seismic energy yield.
||Extremely rare (Unknown)