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Measuring Earthquake Magnitude

Scientists use seismometers to measure earthquake magnitude.

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Measuring Earthquake Magnitude

Can you read a seismogram?

What information can you pick out of this seismograph? Can you see arrival of the P- and S-waves? How many earthquakes were there? Were there foreshocks or aftershocks? At what times do all of these things happen?

Measuring Magnitude

A seismograph produces a graph-like representation of the seismic waves it receives and records them onto a seismogram (Figure below). Seismograms contain information that can be used to determine how strong an earthquake was, how long it lasted, and how far away it was. Modern seismometers record ground motions using electronic motion detectors. The data are then kept digitally on a computer.

These seismograms show the arrival of P-waves and S-waves

These seismograms show the arrival of P-waves and S-waves. The surface waves arrive just after the S-waves and are difficult to distinguish. Time is indicated on the horizontal portion (or x-axis) of the graph.

If a seismogram records P-waves and surface waves but not S-waves, the seismograph was on the other side of the Earth from the earthquake. The amplitude of the waves can be used to determine the magnitude of the earthquake, which will be discussed in a later section.

Interpreting a Seismogram

The seismogram in the introduction shows:

  • foreshocks.
  • the arrival of the P-waves.
  • the arrival of the S-waves.
  • the arrival of the surface waves (very hard to pick out).
  • aftershocks.
  • the times when all of these things occur.


  • A seismograph records seismic waves on a seismogram. A seismometer is a digital seismic wave recorder.
  • Since S-waves do not travel through liquids, a seismogram with no S-waves is on the other side of the planet.
  • Seismographs yield a tremendous amount of information about an earthquake.


  1. Define seismograph, seismogram, and seismometer.
  2. What does a seismogram with P-waves but not S-waves mean?
  3. How can you tell a main earthquake from foreshocks and aftershocks?

Explore More

Use this resource (start at 06:30) to answer the questions that follow.

  1. What is the Mercalli Scale?
  2. What happens at level IX?
  3. What does the Richter Scale measure?
  4. What is the most damaging quake on this scale?
  5. What does each step of the Richter scale represent in energy increase.
  6. In our historical experience what releases the most energy: earthquakes, volcanic eruptions or atomic bombs?
  7. How often do we have earthquake of magnitude 8 and above?
  8. What does the Moment Magnitude scale measure?
  9. Where is the epicenter of an earthquake?
  10. How do you calculate the epicenter of an earthquake?

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    A seismogram is the printed record of seismic activity produced by a seismometer.


    An older type of seismometer in which a suspended, weighted pen wrote on a drum that moved with the ground.


    A seismometer is a machine that measures seismic waves and other ground motions.

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