Why do the seismic waves follow curved paths? (you can choose more than one)
A. P waves do not travel through liquids but S waves do
B. The denser the Earth the faster the waves travel
C. The more rigid the structure of the Earth the faster the waves travel
D. There is a balance between density and rigidity
Answer
C
| Comments | 2 |
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John
I don't think answer C or the other options are barking up the right tree here.
I think we need to take a step back and explore the background to the (very good) question.
Consider the case of an Earth of uniform composition and constant temperature. Pressure will increase with depth, and so will density, and therefore also seismic velocity. So Snells Law says (not sure if this is part of the KS4 curriculum) that if we treat the Earth as a series of layers of increasing density (and therefore velocity) then the angle of refraction will progressively increase across each layer boundary - hence we get a concave outwards (i.e. towards the Earth's surface) ray path.
This is most easily understood in pictures rather than words: try http://www.iris.edu/hq/programs/education_and_outreach/animations/13
Paul
2
Adam Little
Hi John, would you accompany this question with a picture of the structure of the Earth as well? Can you clarify what you mean by rigidity as with my classes I tended to focus on just density and would be interested to hear what you discuss with your classes.