Seismic Waves

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 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.

Click on the image to see an enlarged version.

Below, some of the major differences between P and S waves are highlighted.

***a.k.a. compressional Waves
***a.k.a. longitudinal waves
***1st to arrive at seismic stations
***travel at 1.5-8 km/sec in the Earth's crust
***shake the ground in the direction they are propagating
***travel through the Earth's core
***a.k.a. shear waves
***2nd to arrive at siesmic stations
***1.7 times slower than P waves
***shake the ground perpendicular to the direction in which they are propagating
***do not travel through liquid (ie. water, molten rock, the Earth's outer core)

Decades ago, seismologists noticed that records from an earthquake changed once the event was a certain distance away, aproximately 105o 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 105o 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 moltne. 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.

Seismographs and Seismograms
The Earth's Inside
Plate Tectonics and Earthquakes