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6.1.1. Seismic discontinuities

Seismic discontinuities

The seismic discontinuities are the areas where abrupt changes in the velocity of the P and S waves

When the density of the Earth was calculated, it was observed that the results showed that the average density of the Earth was much higher than the density of the rocks that are on the surface, so it was clear that the Earth was not homogeneous, and that inside, the density of the materials had to be much higher. In addition, if the Earth were homogeneous, seismic waves would travel in a straight line without changing their trajectory because they did not have to change the medium.

Thus, it has been shown that the Earth is heterogeneous and is made up of concentric layers with different properties.

The main discontinuities in the interior of the Earth are the following:

  • Mohorovicic discontinuity
    • In this area, the seismic waves P and S increase their speed sharply.
    • It separates the less dense materials of the crust (silicates of aluminum, calcium, sodium and potassium) from the densest materials of the mantle (silicates of iron and magnesium).
    • It is located at an average depth of about 35 km, and can be found 70 km deep under the continents or only 10 km under the oceans.
  • Gutenberg discontinuity
    • Indicates the separation between the mantle and the Earth's core .
    • In this area, about 2,900 km deep, the P waves decrease their speed sharply and the S waves cannot pass through it, so the nucleus must be in a fluid state.
  • Wiechert-Lehmann-Jeffrys discontinuity or Lehmann discontinuity
    • Located at an average depth of 5155 km, it separates the outer core (fluid) from the inner core (solid) of the Earth, where there is an increase in the speed of P waves.
    • The center of the Earth is about 6371 km.

It should also be noted that there is another discontinuity, between 100 and 250 km deep, in which there is a low speed zone for seismic waves , important for tectonic movements.

Interactive activity: Characteristics of the layers of the Earth according to the speed of seismic waves.