© 2015 Yiannis Krikis
S Wave Shadow Zone
From and early age, we are all taught that the interior of the Earth can be divided into four distinct layers; a solid crust, a plastic mantle, liquid outer core, and solid inner core. But have you ever wondered how we know these layers exist? Obviously, nobody has every managed to drill down to the center of the Earth to check. The deepest hole drilled so far, the Kola Superdeep Borehole in Russia, is just over 12 km deep, far shy of the 6, 371 km that would be required to reach the center of the Earth.
Most of what we know about the interior structure of the Earth comes from studying how seismic waves generated by large earthquakes move through the planet. By using networks of sophisticated seismometers installed in bedrock around the world, geophysicists can record even the smallest vibrations. Seismic waves that travel through the Earth are generally divided into two types; compressional (P waves) and shear (S waves). P waves compress and expand material in the direction the wave is propagating. S waves shear the material perpendicular to the direction the wave is propagating. P waves travel much faster than S waves, and can be differentiated on a seismograph on the basis of their arrival times. A great way to visualize how these waves move is with a slinky, here’s an example: http://bit.ly/2xXRXgq
Liquids cannot sustain shear forces, meaning S waves can only propagate through solid material. When an earthquake occurs, seismic waves are generated at the hypocenter and propagate in all directions through the Earth. S waves are able to pass through the solid crust and mantle and can detected by seismometers on the other side. When an S wave crosses the boundary between the mantle and the outer core, some of the energy contained in the wave gets reflected back toward the surface, and some is transmitted through the core as a P wave. In the 1930’s, Beno Gutenberg found a lack of certain previously predicted S wave recordings at seismometers located more than 103 degrees of latitude from where the earthquake took place. This is known as the ‘S wave shadow zone’ for a given earthquake, and is directly caused by the inability of S waves to pass through the outer core.