Earthquake Measurement
The severity of an earthquake can be expressed in terms of both magnitude and intensity. However, the two terms are quite different, and they are often confused.
INTENSITY
Intensity is defined as the degree of damage.It is based on the observed effects of ground shaking on people, buildings, and natural features. It varies from place to place within the disturbed region depending on the location of the observer with respect to the earthquake epicenter.
Intensity scale:
- Rossi Forrels scale
- Modified Mercalli Scale
- MSK Scale
- EMS Scale
MAGNITUDE
Magnitude is related to the amount of seismic energy released at the hypocenter of the earthquake. It is based on the amplitude of the earthquake waves recorded on instruments which have a common calibration. The magnitude of an earthquake is thus represented by a single, instrumentally determined value. Richter developed a scale using the amplitudes recorded during earthquake with standard Wood Anderson seismograph in California .Richter magnitude scale is a logarithamic scale .An increase in magnitude by 1 means 10 times higher wave amplitude and about 30 times higher energy release .Earthquake magnitude can be expressed in terms of energy of TNT.
Magnitude and energy :
| Magnitude | Approximate TNT Energy |
|---|---|
| 4.0 | 6 tons |
| 5.0 | 199 tons |
| 6.0 | 6,270 tons |
| 7.0 | 199,000 tons |
| 8.0 | 6,270,000 tons |
| 9.0 | 99,000,000 tons |
Magnitude Scale:
- Richter Magnitude Scale(Ml)
- Surface Waves Magnitude Scale(Ms)
- Body Wave Magnitude Scale(Mb)
- Moment Magnitude Scale(Mw)
- Coda or Duration Magnitude Scale (Md)
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| FIGURE 2 - USE THE AMPLITUDE TO DERIVE THE MAGNITUDE OF THE EARTHQUAKE, AND THE DISTANCE FROM THE EPICENTER |
Seismograph:
| How a Seismograph Works | |
|---|---|
| The study of earthquake waves is called seismology. Seismographs are the instruments used to record earthquake waves. The principle behind how a seismograph works is simple.A weight is freely suspended from a support that is attached to bedrock. | |
When the seismic waves from a distant earthquake reach the seismograph, the inertia of the weight keeps it stationary while Earth and the support vibrate. The movement of Earth in relation to the stationary weight is recorded on a rotating drum. The seismograph amplifies and records the ground motion. This recording of information is called a seismogram and it provides a lot of information about the behaviour of the seismic waves. |
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Seismograms show that two main types of seismic waves are generated when there is a slippage in a fault zone.
These are surface and body waves. |
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