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Geology glossary

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magnitude

Earthquakes
A number that characterizes the relative size of an earthquake. Different scales are used, but the most common is the logarithmic Richter scale. where most quakes are between 0 (tiny) and 9 (extremely large quake). Each step on this scale indicates a 10 fold energy increase of an earthquake. Felt quakes are normally above magnitude 2.5 on this scale.
Magnitude is based on measurement of the maximum motion recorded by a seismograph(sometimes for earthquake waves of a particular frequency), corrected for attenuation to a standardized distance.

Several scales have been defined, but the most commonly used are (1) local magnitude (ML), commonly referred to as œRichter magnitude, (2) surface-wave magnitude (Ms), (3) body-wave magnitude (Mb), and (4) moment magnitude (Mw). ML, Ms and Mb have limited range and applicability and do not satisfactorily measure the size of the largest earthquakes.

The moment magnitude (Mw) scale, based on the concept of seismic moment, is uniformly applicable to all sizes of earthquakes but is more difficult to compute than the other types. In principal, all magnitude scales could be cross calibrated to yield the same value for any given earthquake, but this expectation has proven to be only approximately true, thus the need to specify the magnitude type as well as its value.

An increase of one unit of magnitude (for example, from 4.6 to 5.6) represents a 10-fold increase in wave amplitude on a seismogram or approximately a 30-fold increase in the energy released. In other words, a magnitude 6.7 earthquake releases over 900 times (30 times 30) the energy of a 4.7 earthquake – or it takes about 900 magnitude 4.7 earthquakes to equal the energy released in a single 6.7 earthquake!

There is no beginning nor end to this scale. However, rock mechanics seem to preclude earthquakes smaller than about -1 or larger than about 9.5. A magnitude -1.0 event releases about 900 times less energy than a magnitude 1.0 quake. Except in special circumstances, earthquakes below magnitude 2.5 are not generally not felt by humans.

(Source: USGS-IASPEI
 

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