Seismic Waves
Beneath the Earth's surface there are two layers, the Core and the Mantle.
The core is a very hot metal made up of mostly iron and nickel. Most of it is a liquid, but due to the immensely high pressure, some of it is solid.
The mantle is made up of solid rock, but some of it is molten like lava from a volcano. It can move around the earth in large convection currents.
The crust is the layer on which we live. It is divided into sections, each holding one continent.
We know most of this through various deductions. During an earthquake, three types of waves are produced; P-Waves, S-Waves and Surface Waves.
P-Waves are longitudinal waves (like sound) and travel fast through the core and mantle.
A,B,C,D,Y & X are all seismometer stations - they are all capable of detecting both S and P Waves.
A,B,C & D all detect both S-Waves and P-Waves.
However, at Y & Z, only P-Waves are detected. This is because the S-Waves cannot travel through the Earth's core and are therefore absorbed before they reach those stations.
S-Waves are transverse waves (like light) and they cannot travel through the Earth's liquid core.
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