Ocean Swell as a Source to Probe the Deep Earth : Retrieving core mantle boundary P-wave reflections from a single microseism event

The Core Mantle Boundary (CMB) marks the limit between the turbulent flow of liquid molten iron alloy in the outer core and the viscous silicate rock of the lower mantle. It is, with the Earth’s surface, the region that shows the largest contrasts in density, viscosity, and elastic-wave properties. An accurate picture of this region is critical to better understand past and present dynamics of the Earth. Just above the CMB, the lower mantle is generally associated with a thermal boundary shell produced by the heat flux from the hotter core, which is also influenced by mantle convection, and possibly containing a concentration of dense (subducted) materials. Seismology obviously has a major role by providing direct measurements of the elastic properties of these structures. Two approaches prevail : large-scale tomographic images from travel-times and normal modes measurements, and detailed observations and modeling of various high-frequency seismic phases that cross and interact with specific regions. Moderate to large Earthquakes are usually the only source considered to have the right properties to illuminate these remote region of our planet. Several recent studies have shown that secondary microseism excitation can be use to probe structures as deep as the Earth’s core between pairs of sensors when seismic interferometry is performed on continuous seismic records (Boué et al. 2013, Retailleau et al. 2020). During this internship, the student will explore the possibility to retrieve P-wave reflections at the CMB (PcP) from a single, or a collection of microseism event(s). Robust catalogue of large microseism events can be computed from oceanic microseism source radiation derived from global ocean wave models (WW3, Ardhun et al., 2015). The main idea is to extract new observations for deep that could ultimately complement earthquake data for deep Earth imaging.

References
• Ardhuin et al. (2015). How ocean waves rock the Earth : Two mechanisms explain microseisms with periods 3 to 300 s. GRL 42(3)
• Boué et al. (2013). Teleseismic correlations of ambient seismic noise for deep global imaging of the Earth. GJI 194(2)
• Retailleau et al. (2020). Ambient Seismic Noise Imaging of the Lowermost Mantle Beneath the North Atlantic Ocean. GJI, 222(2)

Important skills and motivations for the candidate :
• Seismology and Signal processing
• Deep Earth structures
• Coding (Python, Matlab...)