Chapter 5

Seismological tomography

“The new thing I’ve gone into with Keith PriestleyKeith PriestleyKeith Priestley (1946-present) of the University of Cambridge is a seismologist with an interest in the structure and mechanical behaviour of the continents. He combines comprehensive seismological information with geochemistry, petrology and mineral physics to delve into the fundamental processes that shape the Earth. is essentially seismic tomography from surface waves,” says McKenzie. “Dealing with these huge datasets particularly with the surface waves is a real undertaking, which Keith does and he does it extremely well, but he doesn’t have the sort of background I have, worrying about how the mantle convects, about continental tectonics, and this sort of thing because he’s a card-carrying seismologist.”

Three-component digital seismometers now record worldwide; all the data is shared and available to download from IRIS in Washington State – unless you need very large amounts. “Keith needs so much that he actually flies out there and brings back a terabyte disc full of data,” notes McKenzie. “The fastest method of transferring data is still a man on a bicycle! I suspect that’s going to remain the same for a very long time.”

“The latest inversion used a million seismograms; that’s about 20 terabytes of data – a very large amount. But the result of all this is that you can really see in detail the deep structure of the continents. Oceans are less interesting, but the continents are really spectacular,” says McKenzie. “This data is available to everybody, different people have done the inversions and the tomography in quite different ways and the agreement between them is really excellent. And that means that one can believe it.”