Download Types of seismic waveS

U. 1: PLATE TECTONICS
How do scientists know about the
Earth's interior?
Methods to study the interior of the earth
1. Mines (but no more than 13 km) and seismic waves
2. Study of rocks
3. Meteorites
4. Seismic methods
Seismic Waves
There are two general categories of seismic waves.
1. P-waves, which are longitudinal pressure waves and can propagate in both solids
and liquids. Primary
2. S-waves, which are transverse waves that can propagate in solids but not in
liquids. Secondary
Seismic waves have their direction of motion changed (refracted) by variations in the
interior density. Thus, by studying the way such waves propagate in the Earth we can
learn something about density variations. Second, the fact that P-waves propagate in
liquids but S-waves do not allows us to determine if portions of the interior are liquid.
Types of seismic waveS
There are many types of seismic waves, body wave, surface waves, S waves and P waves. Other modes of wave
propagation exist than those described in this article, but they are of comparatively minor importance for earthborne waves, although they are important in the case of asteroseismology.
Body waves
Body waves travel through the interior of the Earth. They create raypaths refracted by the
varying density and modulus (stiffness) of the Earth's interior. The density and modulus, in turn, vary according to
temperature, composition, and phase. This effect is similar to the refraction of light waves.
Primary waves
Primary waves (P-waves) are compressional waves that are longitudinal in nature. P waves are pressure waves
that travel faster than other waves through the earth to arrive at seismograph stations first hence the name
"Primary". These waves can travel through any type of material, including fluids, and can travel at nearly twice the
speed of S waves. In air, they take the form of sound waves, hence they travel at the speed of sound. Typical
speeds are 330 m/s in air, 1450 m/s in water and about 5000 m/s in granite. Primary waves also travel about 1 to
5 miles per second (1.6 to 8 kps), depending on the material they're moving through.
Secondary waves
Secondary waves (S-waves) are shear waves that are transverse in nature. These waves arrive at seismograph
stations after the faster moving P waves during an earthquake and displace the ground perpendicular to the
direction of propagation. Depending on the propagational direction, the wave can take on different surface
characteristics; for example, in the case of horizontally polarized S waves, the ground moves alternately to one
side and then the other. S waves can travel only through solids, as fluids (liquids and gases) do not support shear
stresses. S waves are slower than P waves, and speeds are typically around 60% of that of P waves in any given
material.
Surface waves
Surface waves (L-waves) are analogous to water waves and travel along the Earth's surface. They travel slower
than body waves. Because of their low frequency, long duration, and large amplitude, they can be the most
destructive type of seismic wave. They are called surface waves because they diminish as they get further from
the surface
Videos
Seismic waves
Structure of the Earth
The Layers of the Earth
Mantle (geology) - Wiki Article
The Earth's magnetic field is mostly caused by electric currents in the
liquid outer core, which is composed of highly conductive molten iron.
A magnetic field is generated by a feedback loop: current loops
generate magnetic fields (Ampère's circuital law); a changing magnetic
field generates an electric field (Faraday's law); and the electric and
magnetic fields exert a force on the charges that are flowing in currents
(the Lorentz force)
2.THE MOVEMENT OF CONTINENTS
3.THE THEORY OF PLATE TECTONICS
3.1 Seismic and volcanic belts
LITHOSPHERIC PLATES OR TECTONIC PLATES AND TYPES OF PLATE BOUNDARIES