Seismology in the deep mantle: a new structural interpretation

Seismology in the deep mantle: a new structural interpretation

The Earth’s core-mantle boundary (CMB) is located approximately 3000 km below our feet, 270 times further removed from our lives than the bottom of the Mariana trench. Conditions there are hard to imagine: the temperature is almost 4000 degrees, like at the surface of a star; the pressure is over 1.3 million times that of the surface of the Earth; the contrast in material properties is larger than the contrast between solid rock and air. On the mantle side, solid rock flows slowly over geological time scales, ultimately providing the driving forces for plate tectonics. On the core side, fluid iron swirls vigorously, thereby sustaining Earth’s magnetic field. By studying this enigmatic region, we strive to better understand these dynamic processes, leading ultimately to improved constraints on the evolution of our planet.

Due to its inaccessibility, the CMB is primarily studied using seismic waves generated by earthquakes. Combining the signal from different waves generates three-dimensional images of Earth’s interior using techniques similar to those used in medical CT scans. Such images have consistently revealed two large regions, or ‘blobs’ in the mantle above the core, where the velocity of seismic waves is slower. These ‘blobs’ – referred to as large-low-velocity-provinces (LLVPs) – have a significant impact on the dynamics of the mantle and potentially influence plate tectonics. In addition, their presence influences the way in which heat is extracted from the core, thereby altering core flow.

A common aim in lower mantle seismology research is to determine the density structure of the blobs, as density variations are what ultimately drives flow. Previous studies suggested that the LLVPs have a higher density, which requires them to have a different composition to the rest of the mantle. Their higher density could give them a long-term stability, possibly anchoring mantle flow, and providing strong boundary conditions for core convection. However, the robustness of existing density models has been questioned; the uncertainties of the data are relatively large, and the data are influenced by shallower Earth structure.

A study led by Dr Paula Koelemeijer, Junior Research Fellow in Earth Sciences at University College, focuses on the density structure of the lowermost mantle using data from Earth’s free oscillations or normal modes. Paula explains further: “After large magnitude earthquakes of (>7.5 or larger) the Earth behaves much like a bell. The frequencies at which the Earth resonates provides information about its interior,  in the same way the sound of a musical instrument is related to its properties.  By ‘recording’ the sounds of the Earth, we can relate this to structures at great depth inside the Earth.”

The research looked at a special class of Earth oscillations, called Stoneley modes, that are uniquely sensitive to the CMB. By analysing their frequency variations in detail, Paula has shown that the LLVPs have a lower-than-average density, with a large, actively upwelling component. This new interpretation is consistent with other geophysical observations, and provides a new basis for future studies.

The findings have recently been published in Nature Communications, with co-authors Arwen Deuss (DPhil, Univ 1998) and Jeroen Ritsema.

Paper: Koelemeijer, P., A. Deuss & J. Ritsema (2017). Density structure of Earth’s lowermost mantle from Stoneley mode splitting observations. Nature Comm., Vol. 8, 15241, doi:10.1038/ncomms15241.

In press: Paula describes her work further in an article published for The Conversation, also picked up by the Daily Mail.