James Jackson Discusses How the Continents Deform
Interview for the Special Topic of Earthquakes, November 2010
Seismology
is the discipline that interprets data from earthquakes in order
to image structures, sources, and boundaries deep within the
earth. The solid earth has three types of seismic waves: the
longitudinal P-waves, the transverse S-waves, plus interface
waves.
The P-waves (primary waves) arrive first at a seismometer because they are the fastest. An important early discovery in seismology dates from 1926 when Harold Jeffreys at Cambridge demonstrated decisively that P-waves travel through the outer core and S-waves do not, thus confirming the hypothesis that the outer core is liquid.
Our Special Topics analysis on earthquakes research published over the past decade highlights the discoveries Professor James A. Jackson FRS and his colleagues. Jackson's work ranks at #3 by total cites and #20 by total papers, based on 49 papers cited a total of 1,257 times. In Essential Science IndicatorsSM from Thomson Reuters, his papers are in the top 1% in the field of Geosciences. Jackson is head of earth sciences at Cambridge University.
James, you've advanced your geophysics career at
Cambridge by using earthquake source seismology to examine how
continents deform. How did you get into this field?
I graduated in geology at Cambridge, and then studied for my Ph.D. under the supervision of Dan McKenzie FRS. Dan is the foremost geophysicist of his generation, and his leadership has helped make geophysics at Cambridge absolutely world class. I joined the laboratory in 1976, after the discovery of plate tectonics in the late 1960s.
The big question at that time was why plate tectonics did not work on the continents. It couldn't explain the structures and landscapes of interest to geologists on land, which is one reason why geologists never discovered plate tectonics: it is an accurate description of the oceans, not the continents. The question then became, what did work on the continents? My approach to finding out how the continents deform was to look at earthquakes.
When you started, was earthquake seismology
important in Cambridge?
Not as important as it is now, Simon. Back then no one here was using earthquakes in that way. The first thing I had to do was to learn earthquake seismology, which I studied at the Seismic Discrimination Group at MIT and in the field, in countries such as Iran and Turkey. My technique was to look at seismograms to interpret the faulting in earthquakes.
Most of the papers in the Special Topics analysis are essentially about using seismograms to infer how faults move. This all happened at a formative time. Fast computers became accessible, and that meant we could use synthetic seismograms to model the waveforms. From this trickery one could tell the precise orientation of the fault in space, the direction in which it moved, and how deep it was.
Photo 1:
"The depth at which earthquakes occur correlates with
the strength of the material. In India, which is manifestly strong,
earthquakes continue right throughout the crust whereas in Tibet they do
not."
On fieldwork in Lake Urumiyeh, Iran. Photo by James
Jackson. View larger image in the tab below.
The new diagnostics had an accuracy that suddenly made seismology relevant to structural geology. Professor McKenzie encouraged me because he could see that this was the right way to go.
What is the unifying theme in the papers we have
selected for this interview?
The intellectual puzzle concerns the continents: Why are they so different from the oceans? Why doesn't plate tectonics work for continents, and how do we use fieldwork and earthquakes to discover what's going on now?
In common with other disciplines, the big changes in research in the earth sciences are often linked to technological advances. We achieved a step change in what we could achieve with seismograms. Since then we've added GPS, allowing us to measure routinely velocities of 1 mm per year all round the globe. We can see what's moving, and, what's not moving at all.
A more recent development is radar interferometry: using satellites, we can image the land surface before and after earthquakes, and see how it has moved. We link these technological advances with actual field investigations: to compare them with what we see on the ground. The movements on faults in earthquakes are actually quite small. The largest you'll ever see is only a few meters. But repeated earthquakes on the same fault create the landscape, so if you understand what happened in one earthquake you can then do the thought experiment and say: "What would happen if we had, say, 1,000 earthquakes?"
That's what I do: visualizing geology in action. So I'm fascinated by questions such as how did the North Sea form, how are mountain belts formed, and so on. That's been the theme running through the whole of my career, and it's a theme that is picked up in your decadal survey of my papers.
I want to start with a popular review paper you
had in GSA Today (12[9]: 4, September 2002). This asks the
intriguing question: "Time to abandon the jelly sandwich?"
The paper examines the issues relevant to understanding why the continents are different from the oceans. For example, here is a first-order question: India has collided with Asia, which has crumpled up all the way from the Himalayas to Mongolia, whereas India is fine (largely unaffected), so what does this tell us?
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