Seismic Record UnitSeismic Record Unit
When you break an object (say a plastic ruler) there is a crack or sound waves moving through the air. Also when we throw a stone into a pond waves also occur (in this case small waves) that spread from where the stone fell into the water banks.
Something similar happens with earthquakes: the rock breaking waves are generated which propagate through the Earth, both within and across its surface. There are basically three types of waves. The first one, called P waves, is the transmission of compressions and rarefactions of the rock, similar to the propagation of sound (figure A). The second type, or S waves consists in the propagation of waves of shearing, in which the particles move in a direction perpendicular to the direction of propagation of the disturbance (figure B.) These two types of waves can propagate inside the Earth and we refer to as body waves.
ONDA S (Figure B) Image © 2000-2006 Lawrence Braile, Used with permission.
There is a third type of waves, called SURFACE WAVES because only propagate through the upper layers of the Earth, its amplitude decreasing with depth. Within this type of waves can be differentiated into two forms, called Rayleigh waves and Love waves in honor of the scientists who demonstrated theoretically its existence.
Rayleigh waves are formed on the surface of Earth and cause the particles move as a retrograde elliptical path (Figure D). Instead Love waves originate at the interface of two media with different mechanical properties, in this case the movement of particles perpendicular to the direction of propagation of disturbance, similar to S waves, but only occurs at the level of Earth's surface (figure C).
less than 85> Love ONDA ( Figure C) Image © 2000-2006 Lawrence Braile, Used with permission.
ONDA R AYLEIGH (Figure D) Image © 2000-2006 Lawrence Braile, Used with permission.
Within this range of waves, P are propagated more rapidly (hence its name, primary ), also presenting feature can be spread by any kind of material, either solid or liquid. S waves travel at a speed slightly lower ( secondary ) and do not propagate through liquid mass. Finally, the surface waves travel at a speed even lower. We can observe these differences in speed in the following seismogram (figure E).
Seismogram (Figure E ) Image © 2000-2006 Lawrence Braile, Used with permission.
A seismogram is a record which represents the movement of soil, measured by a seismograph.
Due to the difference in speed of each type of wave, when we feel an earthquake shakes the first are due to P waves, with the following S waves and surface waves finally. The different rates of each type of wave is also the property that is used to determine the location of focus of the earthquake.
A special case of waves are caused where the focus lies under the sea. This case is very similar to the example of the falling stone in a pond: they generate large waves that propagate from the source to the coast, where they cause serious damage. They are the tsunamis. Perhaps the most notorious example is the earthquake in 1755 in the Atlantic Ocean: the waves reached the coast of Portugal, causing great casualties. Fortunately, these waves are uncommon, require that the sea is sufficiently deep and the earthquake originates is large.
