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Lessons / Restless Earth

Restless Earth · 4 min · 4 steps

Earthquake early warning

Plays here

Goal. Explain P- and S-waves and how an early-warning system wins a few seconds.

Steps

  1. Predict

    When an earthquake starts, which waves reach you first?

    • The strong side-to-side shaking
    • A faster, weaker push-and-pull wave
    • Both at the same time
    Show the answer

    A faster, weaker push-and-pull wave. The faster P-wave, a push-and-pull wave. The stronger S-wave follows.

    This is the largest earthquake near Taiwan in the past week, or a recent one elsewhere if there was none. Its card shows when its waves would reach you: first the P-wave, at about 6 km a second, then the slower S-wave, at about 3.5 km a second, which brings the strong shaking.

  2. Seismometers close to the epicentre catch the P-wave within seconds. Computers estimate the earthquake's size and send an alert by phone and broadcast before the S-wave reaches places further away. Taiwan, Japan, South Korea, Mexico and the west coast of the USA are among the places that run such systems.

  3. 100 km away, the S-wave arrives about 29 seconds after the earthquake starts. If the alert goes out 10 to 15 seconds after the start, people there get about 15 seconds' warning. Close to the epicentre there is little or none: a 'blind zone'.

  4. When the alert sounds or the ground starts to shake: drop to your hands and knees, take cover under a sturdy table and hold on until the shaking stops. With no table, crouch by an inside wall and protect your head and neck with your arms.

The common mistake

Myth

Scientists can predict earthquakes days in advance.

Fact. No one can predict the time, place and size of a particular earthquake. Early warning is not a prediction: it detects an earthquake that has already started and races its alert ahead of the slower, stronger waves.

Check yourself

Question 1

Why can an alert arrive before the strong shaking?

  • Phone and radio signals travel far faster than earthquake waves
  • Animals sense the earthquake first
  • Scientists know the date in advance
Show the answer

Phone and radio signals travel far faster than earthquake waves. The alert travels at nearly the speed of light, while the S-wave travels at about 3.5 km a second.

Question 2

Why might people very close to the epicentre get no warning?

  • The strong waves reach them before an alert can be sent
  • Their phones are switched off
  • The P-wave misses them
Show the answer

The strong waves reach them before an alert can be sent. It takes a few seconds to detect the earthquake and send the alert; nearby, the S-wave arrives sooner than that.

Question 3

What should you do when the ground shakes?

  • Run outside at once
  • Drop, cover and hold on
  • Stand in a doorway
Show the answer

Drop, cover and hold on. Most injuries come from falling objects. Drop, take cover under sturdy furniture and hold on; in modern buildings a doorway is no safer than anywhere else.

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