How do seismic waves indicate whether regions within' the earth are solid or liquid?

Answer 1

Xeno's answer is right but missing one important point. Yes, seismic (basically sonic) waves do travel at different velocities through materials of different densities. But we also get important information from the different components of the seismic wave. Sonic waves are composed of both a compressional (called a P) wave and a shear (or S) wave. Both types travel at different velocities depending on the material they are traveling through but shear waves are not able to travel through a liquid. The shear wave has more of a side to side shearing motion rather than the pushing type motion of a compressional wave. A liquid can't transmit this shearing motion. So, when the receivers pick up only compressional waves we know that a liquid was encountered.

Answer 2

Seismic waves will pass through a solid faster than a liquid. Think of sound waves travelling through air or water. The denser something is the faster it will conduct kinetic energy. The velocity of a seismic wave will alter according to the density of the medium in which it is travelling. Also if an object is too dense refraction and reflection can occur.This has given us the hypothesis that the earths core is made of solid iron and not cheese.