Question

Where did we get our evidence about the Earth's structure?

Answer 1

From p waves and s waves

Explanation:

P waves and S waves are types of siesmic waves generated by the movement of techtonic plates.They can also be artificialy generated by exploding bombs.These waves have different properties.For example. p waves can pass through all mediums: solids,liquids and semi solids while s waves cannot pass through liquids.Thus they create certain 'shadow zones' where no siesmic waves are found.

http://www.visionlearning.com/en/library/Earth-Science/6/Earth-Structure/69

They also are different in their method of propogation and shape.

http://byjus.com/physics/p-wave/

Thus by knowing the rate of slowing down of the waves in materials tested on earth, the composition can be known by doing a direct comparision.

Also by knowing the angles of p and s wave shadow zones and then doing a little complicated geometry, the radius of each of the layer and thus the thickness can be found