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Lessons / Restless Earth

Restless Earth · 4 min · 4 steps

Magnitude: one step is a big step

Plays here

Goal. Understand that magnitude is logarithmic, and tell magnitude apart from the shaking felt at a place.

Steps

  1. Predict

    A magnitude 7 earthquake releases about how much more energy than a magnitude 6?

    • About 17 % more
    • About 10 times more
    • About 32 times more
    Show the answer

    About 32 times more. About 32 times more energy, with waves about 10 times bigger on a seismograph.

    Each earthquake's label shows its magnitude; zoom in to see more labels. The scale is logarithmic: each whole step means waves about 10 times bigger on a seismograph and about 32 times more energy.

  2. So a magnitude 8 releases about 1,000 times the energy of a magnitude 6 (32 × 32). One great earthquake can release more energy than thousands of smaller ones together.

  3. Magnitude is one number for the whole earthquake. The shaking at a place, called intensity, also depends on distance, depth and the ground: soft soil shakes more than rock. Taiwan's Central Weather Administration reports intensity on a scale from 0 to 7, with levels 5 and 6 split into weak and strong.

  4. Worldwide there are roughly 15 earthquakes of magnitude 7 to 7.9 a year, and about one of magnitude 8 or more.

The common mistake

Myth

The magnitude scale stops at 10, and a magnitude 7 is just a bit stronger than a 6.

Fact. The scale has no top: the largest possible earthquake is limited by the size of the fault that breaks. The largest ever recorded, in Chile in 1960, was magnitude 9.5. Each step is about 32 times the energy.

Check yourself

Question 1

How many times more energy does a magnitude 8 release than a magnitude 6?

  • 2 times
  • About 64 times
  • About 1,000 times
Show the answer

About 1,000 times. Each step is about 32 times the energy, and 32 × 32 is about 1,000.

Question 2

Two towns feel the same earthquake. Why might one shake much more?

  • It is closer, or built on soft ground
  • It had a higher magnitude
  • It is further from the epicentre
Show the answer

It is closer, or built on soft ground. An earthquake has one magnitude, but shaking (intensity) is stronger close to it and on soft soil.

Question 3

What limits how big an earthquake can be?

  • The number 10 at the top of the scale
  • The size of the fault that slips
  • The depth of the ocean
Show the answer

The size of the fault that slips. The longer and wider the fault area that slips at once, the bigger the earthquake.

Words in this lesson

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Curriculum

Taiwan 12-year curriculum (108 課綱), natural science
EMd-Vc-4
US Next Generation Science Standards
MS-ESS3-2
England national curriculum
GCSE Geography: tectonic hazards