Revues

Magneto-Coriolis (QG-MC) waves are considered an important part of the rapid dynamics of the Earth's outer core.The detailed characteristics of these waves are however still under scrutiny.In this study we explore the sensitivity of the QG-MC waves to the background magnetic field over which they propagate and the frequency of a periodic perturbation that we impose.We retrieve QG-MC modes by analysing the velocity fields, where they are most easily observed.Concentrations of QG-MC waves in the magnetic field at the core surface in our model are reminiscent of recently observed geomagnetic jerks.The QG-MC waves are weakly sensitive to the details of the background magnetic field during their travel in the bulk and their frequency at the core surface remains close to that of the initial perturbation.This is a potential asset for the prediction of their evolution.Moreover, the waves in the system exhibit a complex relation with the initial perturbation: when the frequency of the initial pulsation is greater than a threshold -- depending on the Alfvén speed of the medium -- inward QG-Alfvén waves are recovered at the core mantle boundary instead of QG-MC waves, and we find that the waves evolve from QG-MC to QG-Alfvén waves depending on the input frequency.Thus, gradually increasing the input frequency in the system, we retrieve the dispersion relation for QG-MC waves with an evolution from a k_s^4 slope to a k_s^1 slope, where k_s is the cylindrical radial wavenumber, as waves transition from QG-MC to QG-Alfvén waves.We actually recover all the components of the dispersion relation from QG-MC waves at low pulsation \omega to QG-Alfvén and inertial waves at high pulsation \omega.Applying our results to the Earth's core, we expect to be able to recover QG-MC waves with confidence in the Earth core with periods between 57y and 2.8y.

Olivier Barrois

Thin anomalous structures known as ultra-low velocity zones (ULVZs) have been found on the core-mantle boundary (CMB) and have extreme velocity reductions. These features are detected due to their effect on seismic waves that travel through them, typically producing precursors or postcursors. In this study we use postcursors to shear core-diffracted waves (Sdiff+) that sample the CMB near Vanuatu to detect and characterise the properties of a ULVZ. We identified a total of 19 earthquakes originating from the South Pacific Rise region detected by stations across East Asia — particularly Japan — showing Sdiff+ signals. Of these events, six with the highest quality Sdiff+ signals are included in a Bayesian inversion of travel times using the 2D Wavefront Tracker we previously developed. A subset of events was selected for further analysis by modelling using 3D full waveform synthetics for a range of parameters. The comparison of the real data with the synthetic waveforms suggests that a ULVZ is located to the southeast of Vanuatu at 172.2±0.9 °E and 22.9±1.1 °S and its broad-scale structure can be approximated as a cylinder with a height of 20±5 km, radius 240±50 km, and shear wave velocity reduction of 30±5%. These parameters are comparable to other ULVZs previously detected and modelled with Sdiff and Sdiff+. There are appreciable uncertainties in the location along the NW-SE direction due to the distribution of earthquakes and seismic arrays, as well as trade-offs between the height, size and velocity reduction of the ULVZ. Other studies using SPdKS, ScP and PcP have reported detections of ULVZs in the proximate region, some of which are consistent with the well-fitting parameter space of the ULVZ in this study. The Vanuatu ULVZ lies within the southwest edge of the Pacific large low velocity province. There is potentially a mantle plume rooted by this ULVZ that has diverted towards the hotspots on the eastern Australian plate around the Tonga slab, although most tomographic models do not show a continuous plume here. 

Carl Martin