Breaking New Ground in Earth Sciences

Interview with seismologist Arwen Deuss

25.10.2012 | Dr. Arwen Deuss, lecturer in theoretical geophysics at the University of Cambridge, made a groundbreaking discovery as an undergraduate that kicked off a career defined by taking new approaches to studying the inner core and the mantle of our planet.
AcademiaNet: As an undergraduate at Utrecht University in the Netherlands, you discovered shear waves in the inner core of the Earth. What was the significance of this finding?

Arwen Deuss
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Arwen Deuss
Arwen Deuss: The inner core of the Earth was discovered in 1936 by a Danish seismologist, a woman called Inge Lehmann, by using data based on the measurement of seismic waves. There are two different types of wave that travel through the Earth: compression waves, which will travel through any type of material – solid or fluid – and shear waves, which only exist in materials that are solid. Lehmann found compression waves that had been partly reflected form the Earth's inner core, but this was an indirect detection, she had no way of proving her findings. If you want to prove that the inner core is definitely solid, you need to observe shear waves that have travelled through the inner core.

So people knew for decades that the inner core was solid, but nobody had ever managed to observe a shear wave travelling through it. When I was an undergraduate, I had a different way of looking at it. These shear waves are so tiny that they were difficult to find, so what I did is add up data from all around the globe. Then this really small signal did start to become visible because I had such a large amount of data - and slowly the wave started to appear. This was the first really direct evidence that the inner core is solid. For Inge Lehmann to make her amazing discovery in 1936, especially for a woman to be able to do that in those times, and for me to finish that story by finding the shear wave was very exciting.

You are also doing a lot of work funded by the European Research Council (ERC) on the composition of the inner core. What is different in the work you are doing for the ERC?

In this research we're looking at whole Earth oscillations. If you have a really big earthquake – like the earthquake in the north of Japan last year or the Sumatra event that caused the 2004 tsunami - the whole earth will start oscillating or "ringing" like a bell.

We measure these oscillations because they give us a really good way of making seismic pictures of what the inner core looks like. What we have discovered is that the inner core consists of different parts, so it's not just one sphere that is nicely homogeneous. These different parts have different crystalline structures, and there are some regions that are stronger in crystalline alignment and some that are weaker. One thing we're thinking is that these regions might be linked to places where the magnetic field of the Earth is stronger or weaker.

You have also been doing work on the mantle of the Earth looking at how fast waves travel through discontinuities and what this tells us about the composition of the mantle. I understand you've discovered something quite interesting about the "mid transition zone discontinuity".

That's a discontinuity that is at the boundary of the upper and lower mantle - this "mid transition zone" is at a depth between roughly 410 and 660 kilometres. People had been confused about the discontinuity there, some people thought you could see it and other people thought you couldn't see it. When I started looking at it I found that if you look over the whole Earth and not at just one place, there are places where you can clearly see a very strong discontinuity and places where you don't see it. I also found places where there was not one discontinuity but two.
Seismic waves
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(© Arwen Deuss)

Seismic waves | from around the globe: With these data, Arwen Deuss discovered two mantle discontinuities under Indonesia, labeled "splitting" in this graph.

We thought that was really strange, but then I started reading mineral physics papers. What I realised was that the mineral physicists had been saying for years that there are two different minerals where we have that mid transition zone discontinuity. So it's a really nice story of finding something new in seismology and suddenly realising that it fitted really well with what had already been predicted and seen in the mineralogy of the mantle.

It seems that quite a lot of your work crosses boundaries with other fields, not just mineral physics but also areas like geophysics and geology. Is this multidisciplinary approach important to you in your research?

Well, my speciality is in seismology: the only thing I know lots about and am really a specialist in is seismology. But I find it a bit boring to just do seismology and look only at the different velocity structures in the Earth. So I always like to go and learn about mineral physics and geology and geochemistry and magnetic fields.

I always make sure I do my seismology with all the knowledge I have and make robust seismic pictures of the deep Earth. But at the end of each project I take one or two weeks where I try to see if I can make a connection with another field. So that's what I really like. I think the only way that you can make your research relevant and fit into the larger story is by trying to make these connections.

As well as all your research, you also do a lot of teaching at undergraduate and postgraduate level. Why have you pursued this particular role as well as your research?

If you want a job as an academic in the UK, the only jobs that you can have are lectureships, but that's not the reason why I do it – I really like it. At Cambridge the students are brilliant and I really enjoy working with them, and they ask good questions! They make me think. Also it's a great way to find PhD students. My PhD students really help me with my research - I've got so many ideas that if I had to do all the research myself I would never be able to do everything I want to do. Working with PhD students will improve my research and it will also advance science because we get so much more research done than if I was to do it all on my own. It is also about training young people to become the next generation of scientists, and I very much enjoy seeing them grow and find academic jobs themselves.

Where do you see your research going in the future?

At the moment I'm largely studying the inner core, that's the ERC research. In doing that I've been using whole Earth oscillations and I've mostly been applying this approach to the inner core. But in the process I have developed many new techniques and I will slowly be using those techniques to study the rest of the earth. I've already started to look at the boundary between the core and the mantle with these new techniques, and I plan to use these approaches to look at the mantle as well and unravel the density variations that we think are responsible for plate tectonics and convection of the mantle, and ultimately the occurrence of earthquakes.

Dear Dr. Deuss, thank you very much for this interesting interview.

Interview: Helen Jaques
  (© AcademiaNet)

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