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How is an Earthquake Measured?

Magnitude

Magnitude is related to the energy generated when a fault ruptures and produces an earthquake. There are different ways to determine magnitude. In one method, three characteristics of the fault zone are used in the calculation:

- The area that ruptures during the earthquake.

- The amount of displacement during the earthquake.

- The stifness of the rocks that break.


When we multiply these numbers, we obtain a number called the seismic moment. The seismic moment is then converted into another number called the moment magnitude (or simply magnitude). In our region, we use the duration of the earthquake recording, and the distance of the recording station from the hypocentre to find magnitude e.g. magnitude 5.8. For any given earthquake, the magnitude is a fixed number that does not vary regardless of which island you are located.


Intensity

Intensity scales describe the severity of an earthquake by grading the effects on people, structures and geological formations. Each degree of intensity is described by a Roman numeral, (I, II, III etc.) and the effects of the earthquake roughly double in severity for each one-division increase in intensity. In the Western hemisphere, including the Eastern Caribbean, the most widely used scale is called the Modified Mercalli or MM scale. In the rest of the world an almost identical scale called the MSK scale is more common. 

For any given earthquake, the Intensity may vary depending where you are in relation to the earthquake's epicenter.

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