Looking at data from several stations: Teachers' notes

Students examine data from several stations at the same time, using a 'record section'.

Learning objectives

Students will:

  • recognise that the arrival times of different phases show that P-waves travel faster than S-waves
  • recognise that the pattern of arrival times provides evidence that speed changes with depth

For the additional task:

  • compare values produced by a simple model with known values
  • identify a limit to the use of the model.

Notes

On the record section shown, the x-axis (time) is common for all traces, and the position of each trace up the page (the y-axis) depends on the distance of that recording station from the event.

Answers to questions

Q1 (Which waves arrive first? What do you notice about the difference in arrival times as the distance from the source increases?)

The P-waves always arrive before the S-waves, and the time between these two waves arriving gets bigger the further the station is from the earthquake

Q2 (Why do the P-wave and S-wave arrival times form curves instead of straight lines?)

The curves show that the speed is not steady but increases with depth. (Refraction)

Q3-6 (Find difference in times of arrival at two stations; calculate distance between the stations, then calculate values for speed of P-waves and S-waves; repeat with different pair of stations).

The answers will vary a bit depending on the stations chosen, but P-waves should be about 5-8 km/s , S-waves about 3-4 km/s.

Q7 (Why might there be a difference?)

a) Refraction effect; b) Over shorter distances, differences in arrival time and average speed are likely to be caused by differences in the materials the waves are passing through. Scientists use large amounts of such data to build models of the Earth's structure at smaller scales.

Q8 (Why don't the lines for the direct P-wave arrival and the direct S-wave arrival continue up to the furthest stations?)

The Earth's liquid outer core refracts P-waves strongly and does not allow S-waves to pass through at all.

Answers to additional task questions

The additional task is meant to offer students a chance to compare a model of the graph with known values.

Q1 The graph is a curve because the difference in arrival times increases with distance.

Q2 The graph stops at just beyond 100 degrees, P-waves and S-waves would have to travel through the liquid outer core, so this model doesn't 'work' any more.

Resources needed

Each group will need: