Thread by @IRIS_EPO

Seismic Shadow Zone - How do P & S waves give evidence for a liquid outer core? (THREAD) iris.edu/hq/inclass/ani… #earthquake #seismology @NSF_GEO

The concept of a seismic shadow zone as a global effect of an earthquake is an oversimplification that ignores a broad array of refracted, reflected and defracted seismic waves that propagate through the earth, as shown in this gif.

Let's focus on direct compressional P- and shearing S-waves. These are the body waves that take the most direct routes within the earth and create shadow zones.

An #earthquake sends P waves in all directions, refracting along boundaries with depth causing curving paths.

At the core-mantle boundary P waves are refracted (or bent) inward and they slow down in the liquid. It's this refraction that causes a shadow zone between the angular distances from 104 to 140 degrees from the #earthquake.

S waves travel along the same path as the P waves, but at slower velocity bc they travel in a shearing motion perpendicular to the direction of travel. Bc of this shearing that they are stopped entirely by the liquid core.

This creates an even broader shadow zone everywhere beyond 104 degrees.
It was this lack of S wave arrivals and recognition of the slowing of the P wave velocity that caught the attention of seismologists in 1910.

By understanding how energy is refracted by different materials and pressures, it enabled seismologists to deduce a liquid outer core. By 1913, the P-wave shadow zone between 104 and 140 degrees was refined to define the core mantle boundary.

Remember these show just two of the types of seismic waves that migrate away from an #earthquake.

TEACHERS! You can use real #data to find the seismic shadow zone in class - here's the activity (and we provide the data) - iris.edu/hq/inclass/les… #earthquake @NSTA @NESTA_US @NSF_GEO

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