Geophysicists and planetary scientists need to know the gross physical properties of the planet in order to model Earth processes, like the travel times of disturbances from earthquakes and how the tectonic plates move. Since 1981, they used a model called PREM. Subsequently, improved measurement techniques and accumulated observations provided more and better data to improve it. A collaboration involving ELSI recently built and published a new reference Earth model, SC-REM, that updates PREM. The new structure indicates that the liquid outer core’s is not radially homogenous, and the density jump at the solid inner core is smaller than previously thought.


[image1] Astronomic-geodetic data fit to SC-REM. Tidal response in the form of the degree-2 Love numbers (a) k2 and (b) h2 (describing the deformation of the Earth by solar and lunar tides), (c) mean normalised moment of inertia I/MR2 (R is Earth’s radius), and (d) M mean mass. For comparison, data fit for the radial seismic reference model PREM and other models are also shown. Credit: Geophysical Journal International.


A research team, including ELSI’s George Helffrich (retired), inverted a large set of normal-mode centre-frequencies and quality (attenuation) factors, including astronomic-geodetic data (mass, moment of inertia and tidal response), using self-consistently built models of the radial elastic and anelastic seismic structure of the Earth. The motivation was to use the further accumulation of observational constraints since PREM was built and to capitalise on the methodological advances that reduced the uncertainty in astrometric-geodetic observations to update the model. The result is a model called SC-REM: a self-consistent reference Earth model.


The model’s self-consistency arises from the way it is constructed: a mineralogical model underlies all of the calculated properties from which temperature, density and seismic wave speeds are calculated. In addition, the minerals deform according to a specified viscoelastic, grain-size-dependent model. This captures the deformation of the Earth from seismic time scales (seconds to hours) to decadal time scales. Another useful feature of the model is that it provides uncertainties for density and seismic wave speed at each level. These were unspecified in PREM.


Apart from these usability characteristics, the new model confirms a few features of the structure of research interest. Firstly, the mantle becomes slightly less dense, and the core becomes more dense. The top of the outer core appears to have an anomalous gradient that suggests a change in composition near its boundary with the mantle. Moreover, the density changes at the inner core boundary, where the core solidifies, is smaller than PREM. This probably means that the energy source driving the Earth’s magnetic field must arise by some mechanism other than the inner core’s crystallisation. The model summarises the observational data upon which future researchers may improve our knowledge of the Earth’s internal processes.


[image2] SC-REM model. The blue areas indicate the range of sampled profiles of (a) density ρ, (b) P-wave velocity VP, (c) S-wave velocity VS, (d) shear-wave quality factor Qμ and (e) temperature T. Histograms of sampled seismic properties at selected depth nodes (2400, 4400 and 5700 km), are shown in the insets in (a), (b) and (c). Insets in (c), (d) and (e) show sampled crustal velocity structure, mantle grain-size distribution, mantle potential temperature Tpot and mantle composition (basalt fraction f), respectively. ‘upper’ and ‘lower’ in the inset in panel (d) refers to the upper and lower mantle, respectively. The sampled models are colour-coded using the 25, 50 and 75 per cent credible intervals (ci.). Models are compared to the isotropic preliminary reference earth model PREM (a–d), ‘outer-core-only’ model EPOC-Vinet (a–c), and the peridotitic laboratory-based mantle adiabat of Katsura (2022) (e and solid vertical red line in top inset in e). The vertical red line in the bottom inset in (e) indicates the value of f corresponding to pyrolite. All seismic models refer to a reference period of 1 s. Credit: Geophysical Journal International.


Journal Geophysical Journal International
Title of the paper Self-consistent models of Earth’s mantle and core from long-period seismic and tidal constraints
Authors J. Kemper1, A. Khan1,2, G. Helffrich3, M. van Driel1, D. Giardini1
Affiliations 1. Institute of Geophysics, ETH Zürich, Zürich 8092, Switzerland
2. Institute of Geochemistry and Petrology, ETH Zürich, Zürich 8092, Switzerland
3. Earth-Life Science Institute (ELSI), Tokyo Institute of Technology, Tokyo 152-8550 , Japan
DOI 10.1093/gji/ggad254
Online published date 24 June 2023