High-pressure experiments allow the spread of seismic waves to be observed in the mineral ferropericlase

Earthquake in the laboratory

Simulating the propagation of seismic waves in the earth is now not only possible by means of computational simulations but through he use of dynamic Diamond Anvil Cell developed at PETRAs III beamline P02.2 (picture: © M. Meschede).

Dynamic Diamond Anvil Cell (dDAC) diffraction setup at PETRA III beamline P02.2

Dynamic Diamond Anvil Cell (dDAC) diffraction setup at PETRA III beamline P02.2 consisting of re-designed dDAC (middle front) and fast GaAs LAMBDA detectors (upper left corner). Insert illustrates the pressure path crated in the dDAC calculated from the diffraction data (picture: DESY/H.-P. Liermann).

Scientists have used a new experimental approach at PETRA III to simulate the way in which seismic waves propagate within a sample, while closely observing the changes taking place inside the material itself. In their study, they were able to show how a process occurring in the mineral ferropericlase above a certain pressure influences the speed with which such waves are propagated. This finding could help to map the composition of the interior of the Earth more accurately. The team led by Hauke Marquardt from the University of Oxford and the Bayerisches Geoinstitut, BGI, at the University of Bayreuth is presenting its findings in the journal Geophysical Research Letters.

When scientists want to know more about the materials and processes occurring in the interior of the Earth, they quickly come up against technical and physical limitations. The deepest bore holes only reach down a few kilometres – not very impressive if you are interested in the lower mantle, which only begins at a depth of around 660 kilometres. The processes that occur in this inaccessible region of the Earth’s interior have huge effects at the surface of the Earth, because they might drive plate tectonics. “In order to understand the interior of the Earth properly, it is important to know its precise chemical and mineralogical composition,” explains Marquardt, the principal author of the study. “One way of finding out more about this composition is to observe and measure the propagation of seismic waves, which are produced during earthquakes. Because the speed of these waves inside the Earth depends on the type of material they are travelling through.”

The more precisely the scientists know the properties of certain minerals, the better they can use the measurements of seismic waves to conclude which of these substances occur in the Earth’s interior. They determine the properties of the minerals by simulating the pressure and temperature conditions in the lower mantle and observing how different materials behave. Until now, one of the problems facing such experiments has been that only very short seismic wavelengths (high frequencies) could be used under conditions of high pressure. Since the typical wavelengths of seismic waves are far larger, however, it was impossible to study directly how such waves are propagated through a sample.

A new experimental method now allows seismic waves to be simulated in the laboratory under different conditions of pressure, and to measure their effects on samples with a high temporal resolution. “We simulate the seismic waves by cyclically compressing and relaxing the sample in a controlled manner, at frequencies that are typical of seismic waves. At the same time, we can use X-rays to measure very accurately how the volume of the material under investigation changes during this time,” says Hanns-Peter Liermann from the Extreme Conditions Beamline P02.2 at DESY’s X-ray source PETRA III, where the new experimental set-up was developed and the measurements were carried out.

In their experiment, the scientists working with Marquardt studied more closely a process that occurs in the mineral ferropericlase ((Mg,Fe)O), the second most abundant component of the lower mantle. Beyond a certain pressure, the electrons in the iron that make up ferropericlase shift their position and adopt a more favourable energy state. This in turn leads to changes in various properties, such as the unite cell volume of the mineral. “For the first time, we were able to measure how the bulk modulus of ferropericlase changes during this transition – at seismic frequencies,” says Marquardt. The bulk modulus describes the resistance of a particular material to changes in volume, and this compressibility directly affects the speed of waves through it. “Our results show that the bulk modulus, and with it the speed of the waves, drops dramatically during the transition, thereby confirming previous studies carried out at very high frequencies.” This finding in turn has consequences for the modelling of the interior structure of the Earth, which needs to take into account that the speed of wave propagation in ferropericlase decreases beyond a certain pressure, and hence beyond a certain depth.

Future studies will further refine the measuring method, allowing experiments to be carried out at very high temperatures too, like those encountered deep inside the Earth. Such studies are being funded in part by the German Research Association (DFG) as part of the FOR2440 research group as well as by the Federal Ministry of Education and Research in the joint research (Verbundforschung) project 05K13RF1. “By using our setup to simulate seismic waves, new ways of studying the earth interior are becoming available, which allow the conditions inside the Earth to be reproduced more and more accurately,” emphasises DESY’s Liermann.

(from DESY news)


Elastic softening of (Mg0.8Fe0.2)O ferropericlase across the iron spin crossover measured at seismic frequencies; H. Marquardt, J. Buchen, A.S.J. Mendez, A. Kurnusov, M. Wendt, A. Rothkirch, D. Pennicard, H.-P. Liermann; Geophysical Research Letters, 2018; DOI: 10.1029/2018GL077982