Recordings of seismic waves from earthquakes led to the discovery of the earth's core and eventual maps of the layers of the Earth's inside.
Just as the prism (in the video below) refracts light at its faces
Seismic waves bend, reflect and change speed at the boundaries between different materials below the Earth's surface.
Earthquakes generate three types of seismic waves: P (primary) waves, S (secondary) waves and surface waves, which arrive at seismic recording stations one after another.
Both P and S waves penetrate the interior of the Earth while surface waves do not. Due to this, P and S waves are known as "body waves". Surface waves arrive last and are the least interesting to seismic tomographers because they don't penetrate deep inside the Earth, therfore provide little information about inacessible terrain.
The major differences between P and S waves
ago, seismologists noticed that records from an earthquake changed
once the event was a certain distance away, aproximately 105 degrees
in terms of the angle between the quake and the seismograph at the
center of the Earth.
After such a distance, waves disappeared almost completely until the slow surface waves arrived. The area beyond 105 degrees forms what is called a SHADOW ZONE.
At greater distances some P waves would arrive but no S waves. From this, researchers determined the Earth's core is fluid and molten _could it rather be a gaseous medium?_
S waves do not travel further since they can not spread through liquid & gaseous medium as for the P waves, they travel at a speed comprised between 1.5 to 8 km/sec. ; 1.5 km/sec. is the speed at which sound waves travel through water therefore, if P waves were travelling through a gaseous medium they should travel roughly at a speed of 330 meters/sec. "the speed changes as well with the temperature and the density of the gaseous medium "...
This explains the lack of S waves in the shadow zone and the bending of P waves to form their own shadow zone.
This is just one example of how seismic tomographers used data concerning seismic waves to map the Earth's inside.