EARTHQUAKE MAGNITUDE SCALES

Richter Magnitude
Charles F. Richter introduced the Richter scale in 1935.

The Richter magnitude is denoted as ML. It is also called the "local magnitude." It is based on the maximum excursion of the needle on the "Wood-Anderson seismograph." The Richter scale was intended for southern California earthquakes only.

The Richter scale is a logarithmic scale. The earthquake wave displacement amplitude increases by a factor of 10 for every 1 unit increase of the Richter magnitude.

The same 1 unit increase in magnitude, however, corresponds to an increase of approximately 32 times the earthquakes energy.
The magnitude calculation depends on two parameters:
1. The maximum displacement indicated on the Wood-Anderson seismograph
2. The distance from the focus to the seismograph
For example, a 23 millimeter displacement measured at a station 210 kilometers from the focus would have a value of ML = 5.0.
A maximum displacement of 2.3 millimeters at this same station and distance would correspond to ML = 4.0.
Finally, note that the displacement indicated on the Wood-Anderson seismograph is proportional to, but not equal to, the ground displacement. The sensitivity of the Wood-Anderson instrument must be known in order to calculate the true ground displacement. A typical amplification factor is 2080. The ground displacement is thus much smaller than the displacement indicated on the seismograph.

Other Magnitude Scales

There are several other scales for measuring earthquakes.

The moment magnitude is denoted by Mw. It takes into account both the energy released and the amplitude of a distant earthquake. This scale is intended for severe earthquakes.

The body-wave scale is mb. It is based on the P-wave, which is the primary wave. This scale is intended for deep focus earthquakes.

The surface-wave scale is Ms.

Mercalli Intensity

The Mercalli intensity scale is yet another system for characterizing earthquakes. This scale is based on qualitative observations, such as building damage. The Modified Mercalli Intensity (MMI) scale is given in the following table.
 Scale
 Effects
 I
 Not felt, except by a few under very favorable circumstances.
 II
Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.
 III
Felt quite noticeably indoors, especially on upper floors of buildings, but not recognized as an earthquake by many people. Standing motorcars may rock slightly. Vibration like passing truck. Duration estimated.
 IV
During the day felt indoors by many, outdoors by a few. At night some awakened. Dishes, windows and doors disturbed; walls make creaking sound. Sensations like heavy truck striking building. Standing motorcars rocked noticeably.
 V
 Felt by nearly everyone; many awakened. Some dishes, windows, etc., broken; a few instances of cracked plaster; unstable objects overturned. Disturbances of trees, poles and other tall objects sometimes noticed. Pendulum clocks may stop.
 VI
Felt by all; many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Damage slight.
 VII
 Everybody runs outdoors. Damage negligible in buildings of good design and constructions; slight to moderate in well built ordinary structures; considerable in poorly built or badly designed structures. Some chimneys broken. Noticed by persons driving motorcars.
 VIII
Damage slight in specially designed structures; considerable in ordinary substantial buildings, with partial collapse; great in poorly built structures. Panel walls thrown out of frame structures. Fall of chimneys. factory stacks, columns, monument walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Persons driving motor cars disturbed.
 IX
Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Underground pipes broken.
 X
Some well-built wooden structures remain standing; most masonry and frame structures destroyed with foundations; ground badly cracked. Rails bent. Landslides considerable from river banks.
 XI
Few, if any masonry structures remain standing. Bridges destroyed. Broad fissures in the ground. Underground pipelines total out of service. Earth slumps and land slips in soft ground. Rails bent slightly.
 XII
Damage total. Waves seen on ground surfaces. Lines of sight and level distorted. Objects thrown up and into the air.


References

1. M. Levy and M. Salvadori, Why the Earth Quakes, Norton, London, 1995.
2. B. Bolt, Earthquakes, Freeman, New York, 1988.

For further information, please visit the following web sites:

The Richter Magnitude Scale
Richter Magnitude
SEISMOLOGY/GEOLOGY GLOSSARY

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Please send comments and questions to Tom Irvine at: tomirvine@aol.com
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