Beamlines | I15 Case Study

How do earthquake waves spread?

The impact earthquakes have on the planet’s surface is well documented. However, much less is known about what happens deep in the planet’s interior. An international group of researchers have been using the Extreme Conditions beamline at Diamond Light Source to study the mineral ferropericlase, thought to be the second most abundant mineral in the Earth’s lower mantle, over 300 miles (670 km) below the surface of the planet. Understanding what happens to ferropericlase in the high temperatures and pressures regimes experienced in the lower mantle can provide clues on how earthquake waves propagate. Their research has been published in the journal Science.

In Detail

The Earth’s lower mantle makes up ~60% of the volume of the planet, and it plays a pivotal role in geodynamics. The lower mantle is mainly composed of (Mg,Fe)O ferropericlase and (Mg,Fe,Al)(Si,Al)O3 perovskite or post-perovskite (ppv). Observed variations in how seismic waves travel in different directions in the deepest part of the lower mantle is likely to be the result of elastic shear anisotropy and lattice preferred orientation of the minerals that make up the mantle. cutaway of the earth showing the lower mantle

Recent experiments carried out at Diamond by researchers from the German Research Center for Geosciences (GFZ), the Karlsruhe Institute of Technology, the University of Bayreuth, and Arizona State University have shown that ferropericlase is likely to be the dominant contributor to anisotropy, even though it composes only ~20% in volume of the lower mantle.

The group used Diamond-Anvil Cells to simulate the high pressures in the deep interior of the Earth and conducted X-ray diffraction measurements at the Extreme Conditions beamline at Diamond. These experiments yielded the high-pressure density of the sample, which is needed to relate measured velocities of sound waves to elastic moduli.

"The dependence of wave velocity on direction increases significantly at a pressure of about 50 Giga-Pascal, corresponding to approximately 1300 km depth and is linked to a change in electronic arrangement (known as a high-to-low spin transition) of the iron ions in ferropericlase. Understanding the properties of materials in the Earth’s mantle means we can derive information about its internal flow from the non-uniform propagation of earthquake waves. This can help to better understand plate tectonic processes.”

Dr Hauke Marquardt, GFZ

Hauke Marquardt, Sergio Speziale, Hans J. Reichmann, Daniel J. Frost, Frank R. Schilling, Edward J. Garnero. Elastic Shear Anisotropy of Ferropericlase in Earth's Lower Mantle", Science, 10 April 2009: Vol. 324. no. 5924, pp. 224 - 226

DOI: 10.1126/science.1169365