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Translations of Encyclopedia about Geology

 

Seismic Waves

In its study of earthquakes and the interior of the earth, the science uses the knowledge obtained by means of measuring devices. The most important one is the seismograph, which registers the waves produced by shocks. It registers both the vertical and the horizontal waves.

During an earthquake, the energy is transmitted to the surrounding rocks. Waves travelling at various speeds reach a seismograph located on the other side of the globe approximately within 20 minutes.

The first ones to arrive are called primary waves and are designated as P-waves. The primary waves move through the rocks and compress them or push them apart. They are called also compressional or longitudinal waves. They travel at a speed of about 5 kilometres per second, which means that they are faster than sound waves travelling through the air.

They are followed by secondary waves, designated as S-waves. These travel through the earth's interior at half the speed, producing vibrations at right angles to their direction of travel. They are also called shear or transverse waves. Waves of both types penetrate through the entire interior of the earth.

Finally, a seismographer registers L-waves, which travel only on the earth's surface. During an earthquake, these waves produce vertical and horizontal motion of the ground. Particles of these Love waves move horizontally. Rayleigh waves travel with a retrograde elliptical motion.

The determination of the earthquake epicentre is based on the time difference between the registration of P-waves and S-waves, in a manner analogous to locating a storm by calculating the time interval between a lightning and the thunder that follows.

The basis for locating the earthquake hypocentre is a travel-time curve. When different measuring sites recorded their data, the computer will determine the exact location of the epicentre, the time the earthquake started, and the depth of its hypocentre. Measuring of the quake intensity is based on the seismic waves, which are transmitted to Richter or Mercalli scale.

Seismograms provide other important data concerning the motion processes in the earth's interior (horizontal or vertical movements).

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