How are seismic waves used to determine the structure of the earth?
Because of the manner of travel of the seismic waves.
First of all, we need to remember that:
P-waves travel through solid, liquid, and gas.
S-waves travel trough solid only.
L-waves travels from focus to directly to the epicenter.
P-waves and S-waves helped scientist to determine the structure of the earth. For example in the core:
At the depth of about