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It is quite apparent that it¡¯s very necessary for scientists and seismologists to be able to measure the magnitude of earthquakes with some kind of scale, so that makes it a lot easier to analyze the level of destruction any particular earthquake possesses. When you saw the heading of this section, you might be wondering why the two terms ¡°magnitude¡± and ¡°intensity¡± are redundantly put beside each other since they are synonyms. However, in seismology, these two terms have quite different meanings.
The sizes of earthquakes vary enormously; the amplitudes of the ground motions vary by a factor of thousands for different earthquakes. Therefore, in order to represent this vast range of magnitude, the only mathematical relationship that can perfectly used here is the logarithmic scale. The scale, which measures the relative strength of an earthquake, is called the Richter Scale, which was devised by an American seismologist called Charles Richter in 1935.
The Richter Scale measures the amplitude of the largest recorded wave recorded during the earthquake. Therefore, by definition, the magnitude of an earthquake is ¡°the logarithm of base 10 of the maximum seismic wave amplitude (in thousandths of a millimeter) recorded on a standardized seismograph at a distance of 100km from the earthquake epicenter¡±[1].
The Richter Scale of common earthquakes falls into the values between 1.0 and 10.0. However, it is also common for some very extreme earthquakes to have values outside of this interval. Since the Richter Scale is logarithmic, we can then say that for each increase of 1 in the Richter magnitude, the amplitude of an earthquake becomes ten times greater.
Since the Richter Scale only measures the magnitude of the earthquake, it does not tell us the degree of effects that any particular earthquake brings to us. This draws the line that distinguishes magnitude from intensity.
Another concept for measuring the degree of earthquakes is to look at the amount of energy is released from the seismic waves of an earthquake. Charles Richter, correlated the idea of magnitude and energy of earthquakes, thus devised another formula which associates the magnitude and energy.
Log E = 11.8 + 1.5 M
Where E is the energy released in ergs and M is the Richter Scale for the corresponding earthquake.
If you make a list of earthquakes with magnitude from 1.0 to 10.0 with an interval between each value of magnitude, and use the above equation to calculate the energy released from each earthquake, you can find a relationship between the energy and the magnitude.
The relationship is quite apparent. You can see that for each increase of magnitude of 1 in Richter Scale, the energy released becomes 31 times greater.
The intensity of an earthquake is a subjective and qualitative observation of an earthquake¡¯s effect and destruction to the landscape. The reason that magnitude is unsuitable for such analysis is that with the same strength of seismic wave from an earthquake, it can have drastic different effects to different landscapes depending on various factors. For example, earthquakes with the same magnitude would bring different levels of destruction to a city, based on the city¡¯s geological features, population and others. Therefore, rather than magnitude, the intensity of an earthquake is not based on mathematical calculations; instead it is based on the subjective evaluations from authorities and victims of the earthquakes.
The major way of evaluating the intensity of an earthquake is a scale called the ¡°Mercalli Scale¡±, devised by Italian geologist Mercalli in 1902. The Mercalli Scale is represented with Roman number symbols ranging from ¡°I¡± to ¡°XII¡±.
The chart below summarizes the characteristics of Mercalli Intensity levels.
Mercalli Intensity |
Richter Scale (approximate) |
Description and Characteristic Effects |
Average peak velocity (m/s) |
Average peak acceleration (m/s/s);g=9.8m/s/s |
I |
<3.4 |
Not felt except by a few under especially favorable circumstances. | ||
II |
<4.0 |
Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing. | ||
III |
4.2 |
Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. Standing automobiles may rock slightly. Vibrations like passing of truck. Duration estimated. | ||
IV |
4.3 - 4.8 |
During the day felt indoors by many, outdoors by few. At night some awakened. Dishes, windows, doors disturbed;walls make creaking sound. Sensation like heavy truck striking building. Standing automobiles rocked noticeable. | 1-2 |
0.015g-0.02g |
V |
4.9 - 5.4 |
Felt by nearly everyone, many awakened. Some dishes, windows, and so on broken;cracked plaster in a few places;unstable objects overturned. Disturbances of trees, poles, and other tall objects sometimes noticed. Pendulum clocks may stop. | 2-5 |
0.03g-0.04g |
VI |
5.5 - 6.1 |
Felt by all, many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster and damaged chimneys. Damage slight. | 5-8 |
0.06g-0.07g |
VII |
5.5 - 6.1 |
Everybody runs outdoors. Damage negligible in buildings of good design and construction;slight tom moderate in well-built ordinary structures; considerable in poorly built or badly designed structures; some chimneys broken. Noticed by persons driving cars | 8-12 |
0.10g-0.15g |
VIII |
6.2 - 6.9 |
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, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Persons driving cars disturbed. | 20-30 |
0.25g-0.30g |
IX |
6.2 - 6.9 |
Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; great in substantial buildings, with partial collapse. Buildings shifted of foundations. Ground cracked conspicuously. Underground pipes broken. | 45-55 |
0.5g-0.55g |
X |
7.0 - 7.3 |
Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations; ground badly cracked. Rails bent. Landslides considerable from river banks and steep slopes. Shifted and mud. Water splashed, slopped over banks. | more than 60 |
more than 0.60g |
XI |
7.4 - 7.9 |
Few, if any,(masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly. | more than 80 |
more than 0.70g |
XII |
>8.0 |
Damage total. Waves seen on ground surface. Lines of sight and level distorted. Objects thrown into the air. | around 100 |
around 0.95g |
The above chart comes from Bolt, Bruce A. (1999). Earthquakes - Fourth Edition. New York. W.H. Freeman and Company. Appendix C. Some modifications involved.
[1]: Bolt, Bruce A. (1999). Earthquakes - Fourth Edition. New York. W.H. Freeman and Company. (pg.152)
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