UNIT 7 The Earth’s dynamics Natural Science 2. Secondary Education HOW AN EARTHQUAKE’S STRENGTH IS MEASURED.

UNIT

7The Earth’s dynamics

Natural Science 2. Secondary Education

HOW AN EARTHQUAKE’SSTRENGTH IS MEASURED

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3UNIT

7How an earthquake’s strength is measured

Natural Science 2. Secondary Education

Intensity and magnitude

• An earthquake’s intensity refers to the effects it causes. It is a subjective value measured by the Mercalli scale.

• The magnitude of an earthquake is a measurement of how much energy it releases. It is an objective value measured by the Richter scale.

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The Mercalli scale The Richter scale

Table summarising the scales and observed

damage

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7How an earthquake’s strength is measured

Natural Science 2. Secondary Education

• It has 12 degrees.

The Mercalli scale

• The lower degrees of the scale deal with how the earthquake is felt by people, while the higher levels are related to observed structural damage (collapsed buildings, roads, etc.).

• The Mercalli scale is used to measure the intensity of an earthquake.

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3UNIT

7How an earthquake’s strength is measured

Natural Science 2. Secondary Education

• The Richter scale assigns a numeric value to the energy an earthquake releases.

The Richter scale

• This scale measures the magnitudeof an earthquake.

• It is a logarithmic scale (for each whole number increase on the scale, 32 times more energy is released). It has no upper limit.

• The most devastating earthquakes measure about 9 on the Richter scale. The earthquake that hit Japan in 2011, for example, had a magnitude of 8.8 on the Richter scale.

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3UNIT

7How an earthquake’s strength is measured

Natural Science 2. Secondary Education

EARTHQUAKE CLASSIFICATION SCALES AND DAMAGE OBSERVED

RichterMagnitude

MercalliIntensity

Witness Observations

< 3 I-II Hard to detect.

3-4 II-IIIVibrations similar to the passing

of a lorry are felt.

>4-5 IV-VSmall objects are overturned. Some people

who are sleeping wake up.

>5-6 VI-VIIIt is difficult to stand. There is damage to brick

structures.

>6-7 VII-VIII General alarm. Some walls may fall.

>7-8 IX-XI Massive destruction. Large landslides.

8-9 XI-XIITotal destruction. The ground

moves in waves.

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