How may the study of earthquake waves be used to interpret the earth's internal structure and composition

The focus of any earthquake cannot be found any deeper than about 700km, this suggests that the composition of the earth below this depth is different from above it and so is not adequate for earthquake foci to be located and therefore it must change from a rock that can be easily fractured to a less easily fracturable rock.

When an earthquake occurs, it produces waves that radiate out from the epicentre (The point on the surface directly above the focus). The body waves that are produced go into the earth. P-waves travel fastest with S-waves coming second, at half the speed.

P and S-waves need dense, rigid rock to travel fastest in, as the rock gets less dense, the body waves slow down and when the rock turns to a liquid form, the S-waves cannot travel through and the P-waves are greatly decreased in velocity and refracted. As body waves travel into the earth, they enter the Moho, this area of crust is very dense and rigid and so both waves speed up rapidly. At approximately 50km, the waves suffer a slight decrease in velocity, before beginning to increase in velocity again after about 50km. This shows that there must be a zone of less dense rock between these depths. This has been named the Low Velocity Zone (LVZ). After leaving the LVZ, both body waves steadily increase in their velocity as they get deeper and travel through the mantle. This shows that the rock found in the mantle must be both rigid and very dense to accommodate such velocity increases. To be precise, the densities of the upper and lower mantle are 3.3g/cm³ and 5.4g/cm³ respectively. At a depth of about 1050km there is a less obvious increase in velocity but it does occur. This is due to the fact that the zone between 400 and 1050km has extremely high pressure, which gradually changes the rock composition to less compressible and more rigid material with the same chemical composition. This area is called the transition zone.

By the time the body waves reach the base of the mantle at about 2900km, the P-wave velocity is up at 13.6km s-1 and the rock density is at about 5.4g/cm³. The S-wave velocity is about 7.4km s-1.

At 2900km, the outer core begins. The P-wave velocity takes a nosedive to 8.1km s-1, and the density increases to about 13.5g/cm³, whereas the S-wave velocity drops to 0.

This evidence gives us the knowledge that the core is of a less rigid, or liquid material as S-waves cannot travel through liquid and P-waves are slowed down by the less rigid rock. It is believed that the core consists of iron with about 7% nickel and 6% sulphur. The low-density sulphur is probably concentrated in the liquid outer core. On the other hand, the high density inner core may contain as much as 40% nickel as the P-waves that reach the inner core are allowed to once again increase in velocity to 11.3km s-1.

All this evidence produced by seismic body waves has led geologists to believe that the earth becomes denser with depth but less rigid. The core being the densest zone but the least rigid and the crust being the least dense but the most rigid zone.