To break an object (say a plastic ruler) produced a clicking sound or sound waves that move through the air. In the same way when we throw a stone into a pond also produced waves (in this case small waves) that spread from where it landed the stone towards the shores of the pond.

Something similar happens with earthquakes: breaking rock generated waves that propagate through the Earth, both in its interior as for its surface. There are basically three types of waves. The first of them, called P-waves, consists of the transmission of compressions and rarefacciones of rock, similar to the propagation of the sound (Figure A). The second type, or waves S, consists of wave propagation of shear, where the particles move in a direction perpendicular to the direction of propagation of the disturbance (Figure B). These two types of waves can propagate through the interior of the Earth and we will refer to them as waves of volume.

WAVES OF VOLUME:

P wave (Figure A).

Wave S (Figure B)

SURFACE WAVES

There is a third type of waves, called surface waves since only they spreading through the uppermost layers of the Earth, decreasing its amplitude with depth. Within this type of waves can be differentiated two forms, referred to as Rayleigh waves and Love waves in honor to the scientists who theoretically demonstrated its existence.

Rayleigh waves are formed on the surface of the Earth and make the particles move as a retrograde elliptical trajectory (Figure D). On the other hand Love waves arise at the interface of two media with different mechanical properties; in this case the motion of the particle is perpendicular to the direction of propagation of the disruption, similar to the S-waves, but only occurs in the plane of the Earth’s surface (Figure C).

LOVE wave

R waveAYLEIGH (Figure D)

Within this range of waves, the P are which propagate faster (hence its name, primary), presenting also the feature to propagate through any type of material, whether solid or liquid. S-waves travel at one slightly lower speed (secondary) and are not propagated by liquid masses. Finally, surface waves travel with one lower speed yet. We can see these differences of speed in the following seismogram (Figure E).

WHAT IS A SEISMOGRAM?:

Seismogram (Figure E)

A seismogram is a record where is represented the movement of the ground, measured by a seismograph.

Due to the difference in the speed of each type of wave, when we feel an earthquake the first shocks are due to the P waves, S waves and surface waves finally to be the following. The different speed of each type of wave is also the property that is used to determine the location of the focus of the earthquake.

A special case of waves are which arise when the bulb is located under the sea. This case is very similar to the example of stone falling in a pond: generated big waves that propagate from the focus towards the coast, where they cause serious damage. They are the tidal waves. Perhaps the most notorious example is the earthquake that occurred in 1755, in the Atlantic Ocean: the waves reached the coast of Portugal, causing large numbers of victims. Fortunately this type of waves are rare, requiring that the sea is deep enough and the earthquake that originates them large.