Gravimetry and normal modes

Séverine Rosat - Researcher at the CNRS

Looking inside the Earth's interior

Download the pdf file of my PhD thesis here (thesis in French)


Seismology has enabled the development of various density models of the Earth's interior (for instance PREM model, Dziewonski and Anderson, 1981). However the density is still poorly constrained in the lower mantle and inside the core. The density models can be determined using surface observations of seismic waves that travel through the core and reflect on the inner core, or the free oscillations of the Earth. These free oscillations, or normal modes of the Earth, give us a direct image of the density structure inside the Earth. The normal modes correspond to a global deformation of the Earth that vibrates at different frequencies, like a bell, after a strong excitation (usually an earthquake of magnitude greater than 6.5). The analysis of the long-period normal modes can constrain the 3D density structure inside the Earth's mantle and core and their frequency splitting, due to the Earth's rotation and ellipticity, is directly linked to the density profile inside the Earth. The translational modes of the inner core, the so-called Slichter triplet (Slichter, 1961), are fed back by buoyancy forces so their period is directly linked to the density jump at the inner core boundary (ICB). The detection of the Slichter mode is therefore primordial as it constraints the density jump and viscosity as well as the stratification of the liquid outer core at the ICB. As a consequence, its detection can not only help us to improve the Earth's density models but also to constrain the quantity of energy required to maintain the geodynamo process through the compositional convection linked to the growth of the inner core. The age of the inner core can therefore also be determined. The stake of the Slichter triplet is multi-disciplinary and primordial.

NEW! Seismic modes after the 2011 Mw9 Sendai earthquake

  • Seismic modes after the 2011 Mw9 Sendai earthquake at Strasbourg SG site

  • Some other results

    Seismic modes after the 2010 Mw8.8 Chile earthquake

  • Seismic modes after the 2010 Mw8.8 Chile earthquake at some SG sites

  • Seismic modes after the 2004 Mw9.0 Sumatra-Andaman earthquake

  • 0S2 amplitude spectrum at Strasbourg (France) superconducting gravimeter site

  • 2S1 amplitude spectrum at Strasbourg (France) superconducting gravimeter site

  • Comparison at Matsushiro (Japan) between the SG and STS1 seismometer records

  • 0S0 radial mode amplitudes at SG sites

  • Slichter modes

  • Predicted Slichter mode amplitudes at some SG sites after past major earthquakes

  • Free Core Nutation Resonance

  • Bayesian inversion of the Free Core Nutation resonance parameters from the combination of 7 SGs gravimetric data

  • PhD summary

    Gravimetry is a privileged tool to investigate the Earth's deep interior at long period, while seismology is more suited to the study of higher frequency phenomena. The time-varying surface gravity is continuously recorded by cryogenic relative gravimeters (superconducting gravimeters, SGs) under the framework of the GGP project (Global Geodynamics Project, Crossley et al., 1999; map in Fig. 1). These variations are induced by geophysical phenomena with periods ranging from some minutes to several years. My complete study of the noise level of superconducting gravimeter sites has shown that cryogenic gravimeters become less noisy than seismometers at frequencies below 1 mHz. I have demonstrated their unique contribution to the study of the long-period normal modes through (1) the well-resolved splitting due to the Earth's rotation and ellipticity of the seismic mode 0S2 into five singlets (five spectral peaks), (2) the first observation of the harmonic degree one seismic mode 2S1, after the Peruvian earthquake of June 23, 2001 with a moment magnitude Mw = 8.4. A large part of my PhD work has also been dedicated to the search for the surface gravity effect of the translational normal mode of the inner core, the Slichter triplet, which has never undoubtedly been observed. In the search for the weak surface gravity signal associated with the Slichter mode, I have developed some stacking methods, automatic detection of weak signals and a wavelet-based analysis method that characterizes the causal damped transient oscillations.

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