They are caused when rock breaks or is deformed. When this happens, waves of energy are released and if strong enough, cause the surface of the Earth to tremble noticibly. Many Earth tremors are of such low magnitude they are not noticed by us.

The classic, standard model of seismic waves dictates that seismic waves occur along fault lines.

Why do the plates slip?

Great stress is placed on rocks along these boundaries. If the stress becomes too great, the tectonic plates slide along each other or slip under each other, releasing the built-up energy in waves. Faults are weak and only need a small stress to cause them to move. One possible cause for earthquakes lies deep under the surface. Far below the surface of the Earth, temperatures and pressures are tremendous. These quantities can become so great that one mineral can be transformed into a new, denser mineral. If this process happens over a large enough area, the old area loses its base of support and drops down. This action would cause tremors.

The 'slash' across these crop rows show where a probably fault line lies. (Note how the crop rows are shifted)
 

Seismic waves

Types of Seismic waves

There are several types of seismic waves. The three that you need to know are P-waves and S-waves (both BODY WAVES - travelling through the body of the Earth) and L-waves (which are SURFACE WAVES - travelling allong the Earth's surface).

NB All of them travel faster the denser the matter they travel through (cf. sound in Y8)

- Body waves move out from the focus of the quake in all directions.

Body waves come in two main forms: S and P waves.

P-waves:

P waves are compressional or longitudinal waves; that is, the medium vibrates parallel to the direction that the wave energy is traveling.

A P wave travels fastest and arrives first at a detector. For this reason, these waves are called primary waves (hence the letter "P"). A P wave can travel through liquid and gas.

S-waves

Diagram 2. S-waves - transverse vibrations

An S wave is slower and arrives at the detector second. For this reason, S waves are called secondary waves ('cos they arrive second!). These waves are transverse waves. In transverse waves, the medium vibrates perpendicularly to the direction of enegy travel.

L-Waves

As these travel through the 'crust of the Earth' (lithosphere). I had through that they got their name from that! But they actually get their name from the scientist/mathematician who discovered them! - A.E.H. Love. There is some erroneous information on the WWW about this!

A Love wave is a surface wave. It is the surface waves that are most damaging as they cause the earth's crust to undulate. (R-waves are also surface waves but we don't need to know about them!)

The L waves travel along the surface of the earth from the point directly above the quake or epicenter. If large enough, they may actually cause ripples on the surface. These waves are the ones that cause most of the damage.

 
 
Reflection

Seismic waves, like light or sound waves can be reflected when the waves strike a boundary. In this way, geologists can detect where boundaries between rock lie.

 
Refraction

Geologists also use the idea of refraction to locate rock boundaries. Refraction occurs when a wave passes from one medium to another. The result is that the wave bends. The wave bends toward the normal if the new medium is denser and away from the normal if the medium is less dense.

By knowing the initial direction of the seismic wave and where it ends up, geologists not only learn that a different type of rock layer is present but also learn the relative density of each layer. Geologists can also use this information to tell where fault lines lie. Using these ideas, scientists have learned that the Earth's crust and much of the mantle are solid.

Rays 'bend towards the normal' when they 'slow down'. In solids the denser the material, the faster the vibrations travel. Therefore in A the wave is speeding up and the grey area must be denser rock. In B it is the other way round!

The Earth's structure

Seismic waves also tell us a great deal about the core. Recall that P waves can travel through liquids whereas S waves cannot. When an earthquake occurs, both S and P waves radiate from the focus. Because the rocks get denser as depth increases the path of the waves bends - see the diagram.

The S waves are detected over a little more than one-quarter of the earth's surface (103° to be exact) . Beyond that, no S waves are seen. This tells us then that for some reason, the S waves do not travel through the core. Hence, the core must be made of liquid. A large, quiet S wave shadow zone is created on the other side of the Earth.  

In contrast, the P waves are detected on the opposite side of the Earth as the focus. A shadow zone from 103° to 142° does exist from P waves, though. Since waves are detected, then not, then reappear again, something inside the Earth must be bending the P waves and bending them towards the normal. From this evidence using waves, we can tell that part of the core is liquid (S wave shadow) and part (the inner part) must be solid with a different density than the rest of the surrounding material (P wave shadow zone due to refraction). In actuality, the inner core is thought to be made of solid iron and nickel.

Locating an Earthquake Epicentre

To find the epicenter of an Earthquake you need to use at least three seismograms. Each earthquake centre can work out how long it took for the P waves to reach the detector by looking at the difference in the time it took for the P and S waves to arrive. From that information they can then work out how far away the epicentre is. But they do not know the direction that the vibrations travelled from. So they use the information from three centres to pinpoint the only point the vibrations could have eminated from. Where the three cross is the epicentre

From seismograms you need to be able to determine the arrival times of the p-waves and the s-waves.(Remember that p-waves will always reach the seismograph first because they travel faster and that s-waves only travel through solids). Then calculate the time difference of arrival of the two wavetypes and use that and the speeds of the waves to work out how far away the epicentre must be.

See also:

York University Site - Earth Sciences Virtual Lab and seismic waves

L. Jones (September 2002)