Normal mode sensitivity to Earth’s D″ layer and topography on the core-mantle boundary: what we can and cannot see

Abstract

SUMMARY The core–mantle boundary (CMB) is Earth’s most profound internal boundary separating the liquid iron outer core and the solid silicate mantle. The detailed structure near the CMB has a major influence on mantle convection and the evolution of the core. Seismic observations, such as topography on the CMB, thin ultra‐low velocity zones (ULVZs), seismic anisotropy and the anticorrelation between shear wave and bulk sound velocity heterogeneities have mainly been made using body waves and are still poorly constrained. We investigate the sensitivity of Earth’s free oscillations to these features and specifically show how large individual anomalies must be for them to be observable. In addition, we discuss the possible trade‐offs between these different lowermost mantle structures. Although modes have strong sensitivity to all the structures inserted, the results illustrate the limits of what normal modes can resolve. Our tests show that: (i) Even small scale features, such as ULVZs, with a thickness larger than 19 km can be observed as long as their distribution contains a long wavelength component. (ii) The peak‐to‐peak amplitude of CMB topography has a larger influence than its pattern and has to be smaller than 5 km to fit the data. (iii) The effect of scaling between shear wave velocity and density anomalies is less constrained, but a laterally varying pattern is implied by a simple test, suggesting the presence of chemical variations. (iv) A strong trade‐off exists between anisotropy in compressional wave velocity and incidence angle whereas shear wave anisotropy is less observable. These findings provide valuable information for future normal mode studies on structures in Earth’s lowermost mantle and their trade‐offs.

Density structure of Earth’s lowermost mantle from Stoneley mode splitting observations

“…These larger CMB topography variations are needed to compensate for the lower P-wave amplitude as well as the lower values of R LL . Despite the fact that these topography variations are larger than for SP12RTS, they still obey the criterion of <5 km peak-to-peak topography28. Furthermore, when we set H =−2 (as found for the best-fitting density model for SP12RTS), the highest probability is found for R LL =1.0, that is, still lighter LLSVPs.…”

“…3d–f). These trends are indicative of the trade-off between CMB topography and lowermost mantle density structure28. High probability values are confined to a relatively narrow range of R LL and R SR for large values of H , and this widens for lower values of H .…”

Latitudinal variation rate of geomagnetic cutoff rigidity in the active Chilean convergent margin

“…In the same line, Florindo and Alfonsi (1995), thought that seismic events could be linked to abrupt topographic changes at the CMB, which generated magnetic variations through the mantle. Another possible explanation of this apparent link between magnetic field, cutoff rigidity and geological systems arises from the instabilities in the CMB that are able to produce secular variations in the magnetic field on the Earth surface, and that corresponds to non-dipolar evolution of the geodynamo (Constable, 2007), since the topography of the CMB is significant in subduction zones, where the subducted slabs can generate downwelling or sinkholes in the deeper areas of the mantle and upwelling or outcrops in areas of divergence (Heirtzler, 2002;Koper, 2003;Hartmann and Pacca, 2009;Lassak et al, 2010;Calkins et al, 2012;Koelemeijer et al, 2012;Bayanjargal, 2013;Tarduno et al, 2015;Pavón-Carrasco and De Santis, 2016;Terra-Nova et al, 2016). However, the latest research on magnetic field and seismicity seems to come from the socalled lithosphere-atmosphere-ionosphere coupling (LAIC) (Hayakawa et al, 2015;De Santis et al, 2015Contoyiannis et al, 2016;Potirakis et al, 2016a, b;Oikonomou et al, 2017).…”

Numerical evolution of the asymmetry in the compositionally inhomogeneous lower mantle driven by Earth’s rotation

“…In this way Ra CR changes between 1.7 9 10 3 and 1.7 9 10 4 in different simulations. Based on the results obtained from seismic tomography, thermo-chemical model calculations and normal modes of the Earth (Koelemeijer et al 2012;Ishii and Tromp 2004;Tackley 2012;Galsa et al 2015) b = 2-4 % might be a reasonable value for the LLSVP. The evolution of the equatorial surface elevation shows an exponentional relaxation to an equilibrium state in every case.…”