|
Magnitude and Intensity
Magnitude is
a measure of the strength of an earthquake or strain energy released by it, as determined by seismographic observations.
This is a logarithmic value originally defined by Charles Richter (1935). 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 seems to preclude earthquakes
smaller than about -1 or larger than about 9.5. A magnitude -1.0 event release about 900 times less energy than a
magnitude 1.0 quake. Except in special circumstances, earthquakes below magnitude 2.5 are not generally felt by
humans.
Intensity is
a measure of the effects of an earthquake at a particular place on humans, structures and (or) the land itself. The intensity
at a point depends not only upon the strength of the earthquake (magnitude) but also upon the distance from the
earthquake to the point and the local geology at that point.
Frequency of Occurrence of Earthquakes
| Descriptor |
Magnitude |
Average Annually |
| Great |
8 and higher |
1 ¹ |
| Major |
7 - 7.9 |
17 ² |
| Strong |
6 - 6.9 |
134 ² |
| Moderate |
5 - 5.9 |
1319 ² |
| Light |
4 - 4.9 |
13,000
(estimated) |
| Minor |
3 - 3.9 |
130,000
(estimated) |
| Very Minor |
2 - 2.9 |
1,300,000
(estimated) |
¹ Based on observations since 1900.
² Based on onservations since 1990.
|
|
See also: 