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

Summary

This 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.