Stratification of the Earth’s Outermost Core : are SmKS phases sensitive enough ?

February-July 2020
Laboratory : ISTerre (Grenoble)
supervisors : P. Boué, H-C. Nataf and D. Jault
Contact : Pierre.Boue univ-grenoble-alpes.fr
Prerequisites : Earth sciences or Physics Licence
Important skills and motivations for the candidate : Deep Earth structure ; Seismology and Signal processing ; Coding (Python, Matlab…)
Keywords : seismology ; core ; spectral elements ; SmKS

intership description

The existence of a stratified layer at the top of the outer core would play a significant role on the liquid core dynamics, and thus on the dynamo effect and on the signature of the magnetic field at the Earth’s surface. Its presence also questions its possible origin : primordial stratification during core formation ? Accumulation of light elements through exsolution ? Thermal stratification ?

For these reasons, it is critical to better constrain the top 500km of the outer core in terms of velocity and density variations. Most seismological observations of this region rely on a particular phase type named SmKS from which it is very difficult to extract reliable and measurements. If such stratification exists, are seismic waves really sensitive to it, and with what accuracy ? We are looking at about a 1% variation in terms of velocity in the first hundreds of kilometers below the Core-Mantle Boundary.

The main objective of this work is to revisit relevant methodologies offered by modern seismology to tackle this fundamental question. In particular, the starting point would be to make use of recent spectral element methods for wavefield simulation. An efficient and accurate
forward modeling is necessary to compare with data. Wavefield generated with AXISEM will be compared with conventional synthetics computed from reflectivity method (usually used in the literature, Tanaka (2014)).

Second, we would like to start to explore the large seismological dataset that is today available online through international data centers. The main objective is to measure reliable differential travel-time of SmKS phases following the work (among others) of Tanaka (2007) and Kaneshima and Helffrich (2013). Finding or not finding discrepancies with the reference model would help to address this controversial subject.

References :

  • Tanaka, S., 2007. Possibility of a low P-wave velocity layer in the outermost core from global SmKS waveforms. Earth and Planetary Science Letters 259 (3-4), 486–499.
  • Tanaka, S., 2014. Broadband waveform modeling with the spectral element method on earth simulator for the study of the structure at the top of the earth’s outer core. JAMSTEC Report of Research and Development 19, 1–9.
  • Kaneshima, S., Helffrich, G., 2013. Vp structure of the outermost core derived from analysing large-scale array data of SmKS waves. Geophysical Journal International 193 (3), 1537–1555.

Mis à jour le 18 octobre 2019