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Seismic waves

Earthquakes are natural phenomena that appear with a sudden waggle of the Earth surface. During an earthquake, part of the energy is released through seismic waves, which are the main cause of the surface waggles.

When an earthquake occurs, some of the energy stored into the rocks is released as seismic waves that spread into the Earth. These waves are called body waves.

There are different seismic waves and the two main groups are:

P-waves (or Primary waves), which travel faster than other waves and they spread as sound waves in the air. The mode of propagation of a P-wave is always longitudinal; thus, the particles in the solid have vibrations along or parallel to the travel direction of the wave energy. Essentially, at their passage, the rocks press and expand continually.


S-waves (or Secondary waves) travel slower than the P-waves. The S-wave moves as a shear or transverse wave, so motion is perpendicular to the direction of wave propagation.  Unlike P-waves, the S-waves do not cause volume variations when they pass through the solid and they do not spread into fluids.

 

When P-waves and S-waves reach the Earth surface; a surface wave, slower than seismic body waves, starts to spread concentrically.

There are two main types of surface waves: Rayleigh Waves and Love Waves.

Rayleigh Waves are similar to those waves on the surface of water. They make the soil vibrate on the basis of elliptical orbits that move backward to the wave spread direction.

 

The Love Waves make the soil vibrate horizontally. The particles motion is transversal and horizontal to the waves spread direction.

 

Because of the long duration and large amplitude of the surface waves, they can be the most destructive type of seismic wave.

INDICATION OF THE BEGINNING OF THE BODY AND SURFACE WAVES

In the seismogram below, the initial stage of the P/S- R/L waves has been indicated with red arrows.

The distance between the waves depends on the distance of these waves from the epicentre.

The Primary wave P arrives before S and R/L. The system ALARM SISMA has been conceived with an extremely sensible sensor able to measure the P-waves and send and audible and visual alarm.

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