Seismology

     Seismometers are instruments that measure the movement of the ground. Most commonly associated with earthquakes, Scientists also use this instrument to determine the nature of Earth's inner core.

(http://www.daviddarling.info/images2/seismograph.jpg)

P Waves

P-waves are a type of body wave, called seismic waves in seismology, that travel through a continuum and are the first waves from an earthquake to arrive at a seismograph. The continuum is made up of gases (as sound waves), liquids, or solids, including the Earth. P-waves can be produced by earthquakes and recorded by seismographs. The name P-wave can stand for either pressure wave as it is formed from alternating compressions and rarefactions or primary wave, as it has the highest velocity and is therefore the first wave to be recorded.^[1]

In isotropic and homogeneous solids, the mode of propagation of a P-wave is always longitudinal; thus, the particles in the solid vibrate along the axis of propagation (the direction of motion) of the wave energy.

(https://en.wikipedia.org/wiki/P-wave)

     P waves help determine the make of the inner core because they travel faster when traveling through denser material. This can be calculated by the the following equation known as Birch's Law.

Geologist Francis Birch discovered a relationship between the velocity of P waves and the density of the material the waves are traveling in:

(https://en.wikipedia.org/wiki/P-wave)

(http://www.astro.uwo.ca/~jlandstr/planets/webfigs/earth/images/waves.gif)

S Waves

S-waves, secondary waves, or shear waves (sometimes called an elastic S-wave) are a type of elastic wave, and are one of the two main types of elastic body waves, so named because they move through the body of an object, unlike surface waves.

The S-wave moves as a shear or transverse wave, so motion is perpendicular to the direction of wave propagation. The wave moves through elastic media, and the main restoring force comes from shear effects. These waves do not diverge, and they obey the continuity equation for incompressible media:

The shadow zone of aP-wave. S-waves don't penetrate the outer core, so they're shadowed everywhere more than 104° away from the epicenter (from USGS)

Its name, S for secondary, comes from the fact that it is the second direct arrival on an earthquake seismogram, after the compressional primary wave, or P-wave, because S-waves travel slower in rock. Unlike the P-wave, the S-wave cannot travel through the molten outer core of the Earth, and this causes a shadow zone for S-waves opposite to where they originate. They can still appear in the solid inner core: when a P-wave strikes the boundary of molten and solid cores, S-waves will then propagate in the solid medium. And when the S-waves hit the boundary again they will in turn create P-waves. This property allows seismologists to determine the nature of the inner core.

(https://en.wikipedia.org/wiki/S-wave)

     There is a blind spot where P waves can't penetrate to which is the Earth's core. S waves also can't reach the core because they can't get past the molten outer core, but P waves can reach the boundary of the solid inner core, turn into S waves, and come back out again as P waves. Based on the speed/time it took for these waves to reach the other end, Scientists are able to determine the type of material that makes up the Earth's core.