How do people now about earth layers?

Answer 1

1. From earthquake waves. There are 2 types of waves, p or primary waves and s or secondary waves that are generated when an earthquake occurs. Both types of waves move away from the epicenter of the earthquake in all directions including "down" through the layers of the Earth under the epicenter. Both waves start out at the same time from the same spot, but P waves move through the earth faster, hence the name primary waves. The farther the earthquake monitoring equipment is from the epicenter the bigger the lag time between when the p and s waves arrive. Monitoring stations on the side of the Earth opposite the epicenter do not receive any S waves. This is because S waves only move through solids so when the S waves hit the liquid outer core, the energy from these waves dissipates. When you move out far enough perpendicularly from the epicenter, S waves show up again on monitors. By overlapping these "shadow zones" from multiple quakes around the world, the depth at which the liquid outer core begins can be determined.