Directionality of crystal elasticity offers explanation for variable seismic character of the inner Earth

Jun 13, 2014
Figure 1: The Earth consists of a surficial crust, a hot, viscous mantle of silicate minerals, a liquid outer core of iron and nickel, and a solid inner core. The D” layer occurs just above the core–mantle boundary. Credit: Alfred Baron, RIKEN SPring-8 Center

Seismic studies enable geoscientists to map the Earth's internal structure. Certain seismic observations, however, remain puzzling, such as the unexpected spatial variability in the speed of seismic waves in a thin zone called the D′′ layer at the boundary between the core and mantle (Fig. 1).

Alfred Baron who leads the Materials Dynamics Laboratory at the RIKEN SPring-8 Center, along with Akira Yoneda of Okayama University and colleagues, have now found that these observations can be explained by the structure and orientation of microcrystals that comprise the D′′ layer.

The D′′ layer is composed mainly of magnesium silicate (MgSiO3) microcrystals with a post-perovskite (pPv) structure. Seismological studies have shown that in the D′′ layer beneath the rim of the Pacific Ocean, horizontal shear waves travel faster than vertical shear waves. Beneath the central Pacific Ocean, however, the relative speeds differ, and underneath the Atlantic Ocean they become equal.

The speed of shear waves through a crystal is related to the crystal's elasticity. The researchers therefore measured the elasticity of microcrystals with the pPv structure. As pPv-MgSiO3 is unstable at ambient pressure, a more stable mineral with the same crystal structure, pPv-calcium iridate (CaIrO3), was studied. "Even that easier experiment is challenging," says Baron, "as the very small crystals of CaIrO3 are not amenable to most methods of sound velocity measurement." Fortunately, such measurements are possible using the inelastic x-ray scattering (IXS) spectrometer built by Baron and his colleagues at the SPring-8 synchrotron radiation facility.

Applying an analysis technique developed by co-author Hiroshi Fukui, the researchers were able to measure the different speeds that shear waves travel through pPv-CaIrO3. The results indicate that the elasticity of the pPv structure is strongly directional, suggesting that the different shear wave velocities observed for the D′′ layer are due to regional differences in the predominant orientation of the microcrystals in the layer.

To explain the observed variation, the researchers suggest that when a slab of material moves downward beneath the Pacific rim, it transforms into pPv-MgSiO3 with a crystallographic orientation that allows horizontal shear waves to travel faster than vertical shear waves. As the slab moves under the central Pacific Ocean, it deforms, leading to a change in crystal orientation and a change in the relative seismic velocities.

Baron's laboratory is now using the IXS technique to determine the speed of sound waves in polycrystalline materials and liquid iron alloys under extreme conditions similar to those of the Earth's core.

Explore further: Iron in the Earth's core weakens before melting

More information: Yoneda, A., Fukui, H., Xu, F., Nakatsuka, A., Yoshiasa, A., Seto, Y., Ono, K., Tsutsui, S., Uchiyama, H. & Baron, A. Q. R. "Elastic anisotropy of experimental analogues of perovskite and post-perovskite help to interpret D'' diversity." Nature Communications 5, 3453 (2014). DOI: 10.1038/ncomms4453

add to favorites email to friend print save as pdf

Related Stories

Iron in the Earth's core weakens before melting

Oct 10, 2013

The iron in the Earth's inner core weakens dramatically before it melts, explaining the unusual properties that exist in the moon-sized solid centre of our planet that have, up until now, been difficult to ...

In Japan, seismic waves slower after rain, large earthquakes

Mar 05, 2012

An earthquake is first detected by the abrupt side-to-side jolt of a passing primary wave. Lagging only slightly behind are shear waves, which radiate out from the earthquake's epicenter and are seen at the surface as a rolling ...

Earth's core reveals an inner weakness

Jan 27, 2014

(Phys.org) —The word "core" conjures up an image of something strong. However, new experiments show that the iron found in the Earth's core is relatively weak. This finding is based on x-ray spectroscopy ...

New study reveals insights on plate tectonics

Mar 04, 2014

(Phys.org) —The Earth's outer layer is made up of a series of moving, interacting plates whose motion at the surface generates earthquakes, creates volcanoes and builds mountains. Geoscientists have long ...

New evidence for oceans of water deep in the Earth

Jun 12, 2014

Researchers from Northwestern University and the University of New Mexico report evidence for potentially oceans worth of water deep beneath the United States. Though not in the familiar liquid form—the ...

Recommended for you

NASA's HS3 looks Hurricane Edouard in the eye

12 hours ago

NASA and NOAA scientists participating in NASA's Hurricane and Severe Storms Sentinel (HS3) mission used their expert skills, combined with a bit of serendipity on Sept. 17, 2014, to guide the remotely piloted ...

Tropical Storm Rachel dwarfed by developing system 90E

16 hours ago

Tropical Storm Rachel is spinning down west of Mexico's Baja California, and another tropical low pressure area developing off the coast of southwestern Mexico dwarfs the tropical storm. NOAA's GOES-West ...

NASA ocean data shows 'climate dance' of plankton

20 hours ago

The greens and blues of the ocean color from NASA satellite data have provided new insights into how climate and ecosystem processes affect the growth cycles of phytoplankton—microscopic aquatic plants ...

Glaciers in the grand canyon of Mars?

21 hours ago

For decades, planetary geologists have speculated that glaciers might once have crept through Valles Marineris, the 2000-mile-long chasm that constitutes the Grand Canyon of Mars. Using satellite images, ...

NASA support key to glacier mapping efforts

21 hours ago

Thanks in part to support from NASA and the National Science Foundation, scientists have produced the first-ever detailed maps of bedrock beneath glaciers in Greenland and Antarctica. This new data will help ...

User comments : 0