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Earthquake Magnitude

Another way to classify an earthquake is by the energy released during the event;

this is referred to as the magnitude of the earthquake. While magnitude has been

measured using the Richter scale, as the frequency of earthquake measurements

around the world increased, it was realized that the Richter magnitude scale was

not valid for all earthquakes (it is not accurate for large magnitude earthquakes).

A new scale called the Moment Magnitude Intensity Scale was developed, which

maintains a similar scale to the Richter scale. This scale estimates the total energy

released by an earthquake and can be used to characterize earthquakes of all sizes

throughout the world. The magnitude is based on the seismic moment (estimated

based on ground motions recorded on a seismogram), which is a product of the

distance a fault moved and the force required to move it. This scale works particularly well with larger earthquakes and has been adopted by the United States Geological Survey. Magnitude is based on a logarithmic scale, which means for each

whole number that you increase, the amplitude of the ground motion recorded by

a seismograph increases by 10 and the energy released increases by 101.5, rather

than one (so that a 3 magnitude quake results in ten times the ground shaking as a

2 magnitude quake; a magnitude 4 quake has 102

or 100 times the level of ground

shaking as a 2 magnitude quake (releasing 103

or 1000 times as much energy). For

a rough comparison of magnitude scale to intensity, see Figure 13.8. Why is it necessary to have more than one type of scale? The magnitude scale allows for worldwide characterization of any earthquake event, while the intensity scale does not.

With an intensity scale, a IV in one location could be ranked a II or III in another

location, based off of building construction (ex. poorly constructed buildings will

suffer more damage in the same magnitude earthquake as those built with stronger construction).

13.4 Locating an Earthquake Epicenter

During an earthquake, seismic waves are sent all over the globe. Though they

may weaken with distance, seismographs are sensitive enough to still detect these

waves. In order to determine the location of an earthquake epicenter, seismographs

Magnitude Typical Maximum Modified Mercalli Intensity

1.0 – 2.9 I

3.0 – 3.9 II – III

4.0 – 4.9 IV – V

5.0 – 5.9 VI – VII

6.0 – 6.9 VII – IX

7.0 and above VIII or above

Figure 13.8 | A comparison of magnitude versus intensity scales for

earthquakes.

Author: Randa Harris

Source: Original Work

License: CC BY-SA 3.0

Member since: 2020-02-01

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