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15.3. Earthquakes


An quake or earthquake, is an earth tremor produced by the release of accumulated energy, in the form of seismic waves, in a place in the lithosphere. They are another of the manifestations of the terrestrial internal energy.

The earthquakes are produced by the activity of faults, by friction between lithospheric plates (in converging edges and edges shear or tear), by volcanoes, or can even give rise to them some human activities.

The fracture of the rocks occurs at a certain depth, the more destructive the earthquake is the lower the depth at which it has occurred.

The point where the earthquake occurs is called the hypocenter, while the place on the earth's surface above the hypocenter is called the epicenter.

The earthquakes can produce shifts in the earth's crust, earthwork, tsunamis or volcanic activity.

Seismic waves

The accumulated energy is released in the form of seismic waves that travel through the interior of the Earth and across the Earth's surface. There are different types of seismic waves:

  • Waves P: or P rimarias, are the F irst to propagate.
  • Waves S o S ecundarias are the S egundas propagate.
  • Surface waves: they only move on the surface of the ground. Their joint action is responsible for the disasters produced by earthquakes. There are two kinds:
    • Waves L or Love.
    • Rayleigh waves.

Video: P, S and surface waves.

Video: Seismogram and images of the 1985 Mexico earthquake.

Magnitude and intensity of earthquakes

The magnitude of an earthquake is the amount of energy that is released in the earthquake, so it is an objective data. The Richter scale is often used in the media.

The magnitude is measured by devices called seismographs, which record the waves that reach them and generate graphs called seismograms with which they can determine the magnitude of the earthquake.

But there is also another way to measure an earthquake. The intensity of an earthquake measures the effects that an earthquake has produced on people and things. The intensity, therefore, is a subjective data, since the earthquake does not affect all people in the same way, in addition to the fact that the damage is less as we move away from the epicenter. There are several scales that measure the intensity of an earthquake, such as the Mercalli scale (1902), the most traditional, and the MSK (Medvedev, Sponheuer and Karnik), which is most widely used today .

IGN macroseismic questionnaire: Have you felt an earthquake?.

Animation: Seismograph.

Answer in your notebook

15.4.- Can two earthquakes have the same intensity but different magnitude? What do you think it can depend on?


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