Shadow zone in seismology refers to an area on the Earth's surface where seismographs cannot detect indirect seismic waves from an earthquake. The shadow zone is the area of the earth between 104 and 140 degrees from an earthquake that does not receive any direct P waves. The shadow zone occurs as a result of the liquid core completely stopping S waves and bending (refracting) P waves. Seismographs placed beyond 145 degrees from the epicentre, on the other hand, record the arrival of P-waves but not S-waves. In this article, you will read about the Emergence of shadow zone which is an important part of the Geography syllabus for the IAS exam.

Shadow Zone

• A seismic shadow zone is an area on the Earth's surface where certain types of seismic waves cannot be measured.
• The shadow zone arises due to the interactions of seismic waves with different layers inside the Earth.
• The existence and nature of these shadow zones provide indirect evidence for the composition and state (solid or liquid) of the Earth's interior layers, especially the core.
• The shadow zone is the area of the earth between 104 and 140 degrees from an earthquake that does not receive any direct P waves.
• A seismograph, an instrument used to record the motion of the Earth's surface caused by earthquake waves, does not show the complete behaviour of the earthquake at these locations.
• Shadow zones are classified into two types:
  • S-wave Shadow Zones
  • P-wave Shadow Zones

Shadow Zone – Background

• Seismographs at remote locations record earthquake waves. There are, however, several specific areas where the waves are not reported. This is known as the "shadow zone."
• The crust, mantle, inner core, and outer core are the distinct structures that make up the earth.
• The crust, mantle, and inner core are all usually solid, while the outer core is completely liquid.
• Richard Oldham, a geologist, was the first to demonstrate a liquid outer core in 1906.
• Oldham studied seismograms from several earthquakes and discovered that some seismic stations, notably those 120 degrees away from the earthquake's hypocenter, failed to catch direct S waves.
• The sudden change in seismic velocities of the P waves and the absence of S waves at the core-mantle boundary was first detected by Beno Gutenberg in 1913.

Emergence of Shadow Zone

• The investigation of several incidents demonstrates that each earthquake has a distinct shadow zone.
• It was discovered that seismographs placed within 105° of the epicentre recorded the arrival of both P and S-waves.
• Seismographs positioned beyond 145° from the epicentre, on the other hand, record the arrival of P-waves but not S-waves.
• As a result, the shadow zone for both types of waves was determined as a zone between 105° and 145° from the epicentre.
• S-waves are not received in the zone beyond 105°.
• S-wave shadow zones are significantly larger than P-wave shadow zones.
• The P-wave shadow zone appears as a band around the globe between 105° and 145° from the epicentre.
• The shadow zone of S-waves is not only greater in size, but it also covers somewhat more than 40% of the earth's surface.

P-wave Shadow Zone

P-wave Shadow zone

• P-waves get refracted (or bent) when they encounter the boundary between the mantle and the outer core due to a change in seismic wave speeds.
• This is due to Snell's Law, which states that when a seismic wave collides with a boundary, it will either refract or reflect.
• The P waves refract in this scenario due to density variations, and their velocity is considerably reduced.
• This refraction creates a P-wave shadow zone between about 105° and 145° from the earthquake's epicenter.
• Seismographs located within this shadow zone don't record direct P-waves from the quake.
• P-waves eventually reappear beyond this shadow zone because some of the waves that pass through the core are refracted back out into the mantle.

S-wave Shadow Zone

S-wave Shadow zone

• S-waves don't travel through liquids.
• Since the outer core is believed to be in a molten or liquid state, S-waves that hit the outer core are absorbed and don't pass through it.
• This results in a large shadow zone for S-waves, beginning at roughly 105° from the earthquake's epicenter and extending across the rest of the planet.
• No direct S-waves are detected beyond this point.
• In the shadow zone, S waves that refract to P waves as they enter the outer core and then refract to an S wave as they leave the outer core can be identified.

Shadow Zone

Significance of Shadow Zone

• The existence of shadow zones, notably S wave shadow zones, could have ramifications for volcano eruption mobility all over the world.
• Volcanoes become eruptive when their percentage melt falls below the rheological lockup (percent crystal fraction when a volcano is eruptive or not eruptive).
• Calculating a volcano's percentage melt could aid in predictive modeling and assessing existing and future threats.
• A study conducted in 2021 on an actively erupting volcano, Mt. Etna in Italy, found that S-waves were absent in some areas and significantly attenuated in others, depending on where the receivers were positioned above the magma chamber.

Conclusion

The seismic shadow zone is a region of the Earth's surface where seismographs cannot detect earthquakes after the waves have passed through it. The most well-known shadow zone is caused by the core-mantle boundary, where P waves are refracted and S waves are stopped at the liquid outer core; however, a shadow zone can be created by any liquid boundary or body.