Seismic Shadow Zone: Definition & Overview

Instructor: Jeff Fennell

Jeff has a master's in engineering and has taught Earth science both domestically and internationally.

Seismic shadow zones are areas away from the epicenter of an earthquake that seismic waves are blocked or refracted away from. This lesson will cover seismic shadow zones and discuss what they tell us about Earth's interior.

Definition

Seismic waves are produced by the energy released from an earthquake. As one side of a fault slips past the other, the pressure that had been stored is released and travels radially away from the focus of the earthquake in the form of primary waves and secondary waves. A seismic shadow zone is an area that receives only one type of seismic wave or, in some places, no seismic activity.

There are two main types of seismic waves:

Primary waves or P-waves are compressional waves that move in a forward and backward motion. They are the first seismic wave to be felt or recorded during an earthquake. If you have ever experienced an earthquake, you may remember the first jolt, this is the P wave.

Secondary waves or S-waves are lateral waves that move as a sine wave perpendicular to the forward movement (propagation) of the wave. They are the second seismic wave to be felt or recorded during an earthquake after the primary wave. When compared to P-waves, S-waves are the more destructive of the seismic waves.

Seismic waves
waves

Primary Waves & Secondary Waves

After the 1906 earthquake in San Francisco, seismographs, instruments designed to record seismic waves, were positioned all over the globe to study earthquakes. Scientists were surprised to find that, while most seismographs were able to pick up P-waves, areas between 100 and 150 degrees away from the epicenter on both sides of the globe were seismically silent. Equally surprising, not all Richter scales recorded S-waves. Only near the epicenter were S-waves observed; at farther distances, they were absent.

Richter scale with S & P waves
richter

This baffled scientists, since S-waves were stronger than P-waves. So they should be able to be detected around the globe as well. The area was not constant; each earthquake epicenter had its own unique area of missing S-waves that later became known as a seismic shadow zone.

It was determined that the seismic shadow zones were caused by the inability of S-waves to pass through liquids. This proved to scientists that the interior of the Earth must contain a layer of liquid material. The P-waves were recorded at most seismographs because they have the ability to pass through both solid and liquid layers, but refraction caused them to 'bend' away from areas between 100 and 150 degrees away from the epicenter.

  • S-wave shadow zone - caused by S-waves' inability to pass through liquid.

S-wave shadow zone
s wave shadow

  • P-wave shadow zone - caused by refraction of seismic waves.

P-wave shadow zone
p shadow

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