Discoveries from Seismic Waves
- The interior of the Earth is not directly observable as it is not physically possible to drill that far
- The furthest humans have managed to drill down is 12.2 km - whereas the distance to the centre of the Earth is over 6000 km
- Scientists now agree that the Earth is made up of:
- Inner core
- Outer core
- Mantle
- Crust
- This understanding has been developed from the study of seismic waves
The Structure of the Earth
The Earth is made up of an inner core, outer core, mantle and crust
P-Waves
- P-waves refract as they pass through the different layers of the Earth from the epicentre
- At the boundary between each layer, the waves refract (change direction) because each layer contains rocks with different properties and densities
- This changes the velocity of the wave
- Resulting in the waves taking a curved path through the Earth's centre
- Analysing the paths of P-waves through the Earth showed that the Earth was made up of layers of different densities
P-Waves Travelling Through the Earth
P waves are sound waves that travel through solids and liquids. They are the fastest seismic wave
- Refractions between layers cause two shadow zones, where no P-waves are detected which suggests that the inner core is solid
S-Waves
- S-waves are unable to travel through the Earth’s molten liquid outer core from the epicentre
- This shows that the outer core of the Earth must be a liquid
- The waves follow a curved path due to refraction but they do not cross the boundary between the mantle and the outer core
- This creates a large shadow zone where no S-waves are detected on the surface of the Earth
S-Waves Travelling Through the Earth
Transverse S-waves are unable to pass through the Earth’s liquid outer core so create a large shadow zone on the other side
- On the opposite side of the Earth to an earthquake, only P-waves are detected, not S-waves, this suggests that the outer core of the Earth is liquid
Worked example
The diagram shows some paths taken by P and S seismic waves from the epicentre of an Earthquake at E to three recording stations at positions 1, 2 and 3.
Compare the seismographs obtained at recording stations 1, 2 and 3.
Answer:
Step 1: Describe the seismograph obtained at station 1
- Station 1's seismograph will show:
- Both P and S waves because the waves can travel through the solid mantle
- P-waves before S-waves / P-waves first because P-waves travel faster / S-waves travel slower
- This results in a small time lag between the detection of the P and S waves
Step 2: Describe the seismograph obtained at station 2 and compare to station 1
- Station 2's seismograph will show:
- The same waves and in the same order as station 1
- The time lag will be longer than for station 1 because station 2 is further away from the epicentre (so the waves travel a greater distance)
Step 3: Describe the seismograph obtained at station 3 and compare to station's 1 and 2
- Station 3's seismograph will show:
- Only P-waves (no S waves) unlike stations 1 and 2 because S-waves cannot travel through the liquid molten outer core
- The P-waves will be detected much later than stations 1 and 2 because it is the station furthest from the epicentre of the earthquake
- The amplitude of the detected waves will be much less than stations 1 and 2 because the amplitude of the detected waves decreases as distance from the epicentre increases
Exam Tip
When comparing or contrasting information make sure you use the correct language such as
- unlike
- only
- compared to
- much less than
- greater than
- however
