Discoveries from Seismic Waves (WJEC GCSE Physics): Revision Note
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
Examiner Tips and Tricks
When comparing or contrasting information make sure you use the correct language such as
unlike
only
compared to
much less than
greater than
however
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