How do P & S waves give evidence for a liquid outer core?

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     • Reflection and Refraction
     • Travel Time Curves

Shadow Zones

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Intro to Shadow Zones

The seismic shadows are the effect of seismic waves striking the core-mantle boundary. P and S waves radiate spherically away from an earthquake’s focus in all directions and return to the surface by many paths. S waves, however, don’t reappear beyond an angular distance of ~103° (as they are stopped by the liquid) and P waves don’t arrive between ~103° and 140° due to refraction at the mantle-core boundary.

Quicktime (5.73 MB)

Seismic Shadow Zones vs Light Shadows

The wave properties of light are used as an analogy to help us understand seismic-wave behavior. Most shadows aren't black; that's because light reflects off of nearby objects and the redirected and reduced light energy strikes and brightens the area in shadow. Light can also bend, for example refracting at the water surface illuminating objects below the surface, or appearing to make half-submerged objects bend. Seismic waves also reflect and refract off layers in the Earth.

Quicktime (9.36 MB)

Shadow Zone Rollover

Flash interactive rollover shows the different P and S phases and their respective shadow zones.

Flash (154 kB)

P Phases and the Shadow Zone

Animation addresses 5 common variations of P-type seismic body waves. All are compressive waves that travel through the Earth in all directions away from the epicenter of an earthquake. The different phases show how the initial P wave changes when encountering boundaries in the Earth.

Quicktime (6.31 MB)

S Phases and the Shadow Zone

Animation addresses 3 common variations of S-type seismic body waves. All are shear waves that travel in all directions away from the epicenter of an earthquake. The different phases show how the initial S wave is stopped, or changes when encountering boundaries in the Earth.

Quicktime (4.49 MB)

Animations and videos are made in partnership with Earthscope, USGS, and Volcano Video & Graphics.

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