What is the primary difference in the travel capabilities of P-waves and S-waves?

P-waves are longitudinal and can travel through both solids and liquids. S-waves are transverse and can only travel through solid materials, as liquids cannot support shear stress.

How does the S-wave shadow zone provide evidence for the Earth's liquid outer core?

Since S-waves cannot travel through liquids, the fact that they are not detected on the side of the Earth opposite an earthquake epicenter indicates they have been blocked by a liquid layer (the outer core).

Why do seismic waves follow a curved path as they travel through the mantle?

Waves follow a curved path due to continuous refraction. As density and pressure increase with depth, the velocity of the waves increases, causing them to bend back toward the surface.

What causes the P-wave shadow zone?

The P-wave shadow zone is caused by the sharp reduction in velocity as P-waves enter the liquid outer core. This change in speed causes the waves to refract (bend) at such an angle that they miss a specific belt on the Earth's surface.

What is a common misconception regarding P-waves in the shadow zone?

A common error is thinking P-waves are completely blocked by the core. In reality, they are merely refracted; they still pass through the core but are diverted away from the shadow zone region.

How was the solid inner core discovered if P-waves slow down in the outer core?

Seismologists noticed that some P-waves arriving at the very center of the far side of the Earth arrived faster than expected. This indicated they passed through a high-density solid region (the inner core) where velocity increased.

What error occurs if a student assumes seismic wave speed is constant throughout the Earth?

Assuming constant speed would lead to the incorrect conclusion that waves travel in straight lines. This would make it impossible to explain why waves arrive at distant stations at the times they do.

Define the Mohorovičić Discontinuity (Moho).

The Moho is the boundary between the Earth's crust and the mantle. It was discovered because seismic waves suddenly accelerate when passing into the denser rock of the mantle.

Compare the size of the P-wave and S-wave shadow zones.

The S-wave shadow zone is much larger, covering everything beyond $103^\circ$ from the epicenter. The P-wave shadow zone is a smaller 'ring' or belt between $103^\circ$ and $142^\circ$.

What happens to the velocity of a P-wave as it moves from the mantle into the outer core?

The velocity decreases abruptly. This is because the outer core is liquid, and while P-waves can travel through liquids, they move slower in them than in the highly compressed solid rock of the lower mantle.

Discoveries from Seismic Waves | WJEC GCSE Physics

1. Seismic Waves as Probes

Seismic waves are energy pulses generated by earthquakes that propagate through the Earth's interior, acting like an X-ray for the planet's structure.
Because the Earth's interior cannot be reached by drilling—the deepest boreholes barely scratch the crust—scientists rely on the travel time and pathway of these waves to infer the density and state of matter at various depths.
The two primary types of body waves used for this purpose are P-waves (longitudinal) and S-waves (transverse), each interacting differently with solid and liquid materials.

Cross-section of Earth showing seismic wave paths. S-waves are blocked by the liquid outer core, while P-waves refract through it, creating distinct shadow zones.

2. Wave Behavior: Reflection and Refraction

As seismic waves move through the Earth, they encounter boundaries between layers of different densities and physical states (solid vs. liquid).
At these boundaries, waves undergo refraction, where they change direction and speed; generally, wave velocity increases with depth as density and stiffness increase in the mantle.
This continuous refraction causes seismic waves to follow curved paths rather than straight lines as they travel through the Earth's body.

3. Evidence for a Liquid Outer Core

The most significant discovery from seismic data is the existence of a liquid outer core, evidenced by the behavior of S-waves.
S-waves are transverse waves that require shear strength to propagate; because liquids have no shear strength, S-waves cannot pass through them.
The resulting S-wave shadow zone—a massive region on the opposite side of the Earth from an earthquake where no S-waves are detected—proves that a liquid layer must exist deep within the planet.

4. The P-Wave Shadow Zone and the Inner Core

P-waves can travel through both solids and liquids, but they slow down significantly when entering the liquid outer core, causing them to refract sharply.
This refraction creates a P-wave shadow zone between approximately $103^\circ$ and $142^\circ$ from the epicenter where direct P-waves are not recorded.
Weak P-waves detected within the shadow zone and specific arrival times suggest the existence of a solid inner core, where waves speed up again due to extreme pressure.

5. Key Distinctions: P-Waves vs. S-Waves

Feature	P-Waves (Primary)	S-Waves (Secondary)
Type	Longitudinal (Compression)	Transverse (Shear)
Speed	Fastest; arrives first	Slower; arrives second
Media	Solids and Liquids	Solids ONLY
Shadow Zone	Small ( $103^\circ$ to $142^\circ$ )	Large ( $103^\circ$ to $180^\circ$ )

Understanding these differences is critical for determining the distance to an epicenter and the state of the material the waves have passed through.

6. Exam Strategy & Tips

Identify the Wave: Always check the arrival order on a seismogram; P-waves always appear before S-waves due to their higher velocity.
Shadow Zone Logic: If a question asks for evidence of a liquid core, focus on the absence of S-waves on the far side of the Earth.
Refraction vs. Reflection: Remember that the curved path of waves is due to refraction (bending due to speed changes), while distinct boundaries like the Moho are identified by sudden reflections or velocity jumps.
Sanity Check: If a wave is detected at $150^\circ$ from the epicenter, it must be a P-wave, as S-waves cannot reach that distance through the core.

Discoveries from Seismic Waves

Summary

1. Seismic Waves as Probes

2. Wave Behavior: Reflection and Refraction

3. Evidence for a Liquid Outer Core

4. The P-Wave Shadow Zone and the Inner Core

5. Key Distinctions: P-Waves vs. S-Waves

6. Exam Strategy & Tips

Discoveries from Seismic Waves

Summary

1. Seismic Waves as Probes

2. Wave Behavior: Reflection and Refraction

3. Evidence for a Liquid Outer Core

4. The P-Wave Shadow Zone and the Inner Core

5. Key Distinctions: P-Waves vs. S-Waves

6. Exam Strategy & Tips