Seismic Shadow Zones—Introduction to P & S wave shadow zones (educational)

IRIS Earthquake Science
22 Jan 201802:00

Summary

TLDRThis script explores the concept of seismic shadow zones, emphasizing that they are an oversimplification of the complex seismic wave propagation through Earth. It focuses on P and S waves, explaining how P waves refract at the core-mantle boundary creating a shadow zone between 104 to 140 degrees, while S waves are entirely blocked by the liquid core, creating a broader shadow zone beyond 104 degrees. The recognition of these phenomena in 1910 led to the discovery of Earth's liquid outer core.

Takeaways

  • 🌐 The concept of seismic shadow zones is an oversimplification that ignores various types of seismic waves.
  • 🔍 Focus is on direct compressional P waves and shearing S waves, which are body waves that travel the most direct routes within the Earth.
  • 🌏 Earthquakes send P waves in all directions, which refract along boundaries with depth, causing curved paths.
  • 🔄 P waves are refracted or bent inward at the core-mantle boundary, slowing down in the liquid core, creating a shadow zone between 104 to 140 degrees.
  • 🚫 S waves travel along the same path as P waves but are entirely stopped by the liquid core, creating a broader shadow zone beyond 104°.
  • 🔬 The absence of S-wave arrivals and the slowing of P-wave velocity caught the attention of seismologists in 1910.
  • 🧠 Understanding how energy is refracted by different materials and pressures helped seismologists deduce the existence of a liquid outer core.
  • 📅 By 1913, the P-wave shadow zone was refined to define the core-mantle boundary.
  • 🌈 These shadow zones are just two examples of the types of seismic waves that migrate away from an earthquake.

Q & A

  • What is a seismic shadow zone?

    -A seismic shadow zone is an area where seismic waves, specifically P and S waves, do not reach due to their refraction and reflection at the Earth's core-mantle boundary, creating a zone of minimal or no seismic activity.

  • Why is the concept of seismic shadow zones considered an oversimplification?

    -The concept is an oversimplification because it ignores the complex propagation of seismic waves, including refracted, reflected, and diffracted waves, that travel through the Earth.

  • What are the two main types of body waves mentioned in the script?

    -The two main types of body waves mentioned are compressional P waves and shearing S waves.

  • How do P waves behave at the core-mantle boundary?

    -At the core-mantle boundary, P waves are refracted or bent inward and slow down in the liquid outer core, which causes a shadow zone.

  • What is the angular distance range of the P-wave shadow zone?

    -The P-wave shadow zone is between angular distances of 104 to 140 degrees from the earthquake's epicenter.

  • Why are S waves stopped entirely by the liquid core?

    -S waves are stopped by the liquid core because they travel in a shearing motion perpendicular to the direction of travel, which cannot propagate through liquid.

  • What is the broader shadow zone created by S waves?

    -The broader shadow zone created by S waves is everywhere beyond 104° from the earthquake's epicenter due to their inability to pass through the liquid outer core.

  • What significant discovery did seismologists make in 1910 regarding S waves?

    -In 1910, seismologists discovered the lack of S-wave arrivals, which led to the recognition of the slowing of P-wave velocity and the deduction of a liquid outer core.

  • How did understanding seismic wave refraction help seismologists?

    -Understanding how energy is refracted by different materials and pressures enabled seismologists to deduce the presence of a liquid outer core.

  • What was the significance of the P-wave shadow zone in defining the core-mantle boundary?

    -The P-wave shadow zone, refined to between 104 and 140 degrees, was significant in defining the core-mantle boundary as it helped to understand the Earth's internal structure.

  • What other types of seismic waves are there besides P and S waves?

    -Besides P and S waves, there are also surface waves, which travel along the Earth's surface and include Love waves and Rayleigh waves.

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Related Tags
Seismic WavesEarthquake ScienceShadow ZonesGeophysical StudiesCore-Mantle BoundaryP-WavesS-WavesLiquid Outer CoreSeismologyWave Propagation