So how do we know what's in the centre of the Earth when no one has been there, apart from Rick Wakeman and Doug McLure ?
Well, there are lots of excellent illustrations courtesy of our old friends the USGS, who have an ILLUSTRATED ARTICLE here (a little slow to load sometimes) - below is an example of the sort of thing you can obtain from this site. Of course these are also available in a good old fashioned thing called a textbook.
Teachers wanting a good lesson with excellent resources could try the RICE UNIVERSITY pages which includes this excellent MAP OF EARTHQUAKE FOCI.
Earthquakes are one of the ways that we know the interior structure of the earth. An earthquake generates several different types of shock waves when the rock fractures at the focus or hypocentre (remember that the epicentre is not where the earthquake actually starts, but represents the point on the surface where the shock is felt most strongly as it is directly above the focus, which is likely to be somewhere in the Benioff zone (depending on the plate margin type...)
These move at different speeds, and are also affected differently by the presence of liquid (or at least molten) or solid material.
Surface waves will also be generated by the earthquake.
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