The seismic method

The seismic method is one of the main indirect study methods that allows us to know what the Earth’s interior is like. It is based on the study of seismic waves produced in earthquakes or by controlled explosions.

Seismic waves occur at a point called the hypocenter and travel through the materials that make up the Earth. When the waves reach the earth’s surface (epicenter) they propagate concentrically. As they move away from the hypocenter, the seismic waves attenuate. Analyzing the speed and trajectory of the waves we can know the chemical composition, physical state and structure of the materials that make up the internal parts of our planet.

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Several types of seismic waves are distinguished:

• P or primary waves. They are the fastest waves, the first to be received by seismographs. The particles vibrate in the same direction as the propagation of seismic waves. They are transmitted through solid and liquid media, although they are faster in solid materials.
• S or secondary waves. They are slower than P waves. The particles vibrate in a direction perpendicular to the propagation of the waves. They are only transmitted through solid media.
• L and R waves. They appear when P and S waves reach the Earth’s surface. They are the slowest, the last to be recorded by seismographs. They are the most destructive waves, since they have a longer wavelength. As they are superficial, they do not provide information about the earthquake.

By Francisco Javier Blanco González (Own work) [Public domain], via Wikimedia Commons

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When the waves change medium, they are refracted, changing their trajectory and speed, so by analyzing the difference of arrival at the different seismographs of the P and S waves, it can be deduced what the interior of the Earth is like. The areas where seismic waves change speed, or may even disappear, are known as seismic discontinuities. Thus, thanks to the seismic method, it has been possible to deduce that the Earth is heterogeneous and that its structure is that of different concentric layers with different properties.

Modificado de https://opentextbc.ca/geology/chapter/9-1-understanding-earth-through-seismology/ by Steven Earle used under a CC-BY 4.0 international license

When the waves refract and change direction, seismic shadow areas are produced in which the seismic waves do not reach, which helps us to deduce at what depth this change of direction occurred and the characteristics of these materials.

By USGS, SVG by Vanessa Ezekowitz [CC BY-SA 3.0], via Wikimedia Commons

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Interactive activity: P waves and S waves.

Interactive activity: Intensity and magnitude of an earthquake.

Video: Seismogram and images of the 1985 Mexico earthquake.

Video: Methods for studying the interior of the Earth. Bio [ESO] sfera (18:14 minutes)

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