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Author
Date
2023Type
- Doctoral Thesis
ETH Bibliography
yes
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Abstract
Determining the structure of planets is paramount to the understanding of the origin and evolution of planetary systems. As seismic waves propagate freely through planets and interact with discontinuities and material structure, their study, seismology is the predominant discipline to resolve planetary interiors.
Density, defined as mass per unit volume, is important in that it is the primary seismic parameter that informs us about the material composition of a planet. In addition, gaining insight into a planet's density structure has many implications for geodynamic, geomagnetic and seismic studies. Earth's mantle flow models are governed by these density differences and thus provide a connection between traditional seismic tomographic models and geodynamics. Moreover, the inner core boundary density contrast drives the convection in the outer core, generating the magnetic field through the geodynamo process.
Combining density information with tomographic velocities allows the inference of material properties, such as shear and bulk modulus, that consequently give constrains on the geochemistry. For these reasons, the overarching goal of this thesis is to increase our understanding of density variations.
To achieve this, we use long period seismic signals as they are affected by self-gravitation. This form of gravitation affects waves that, while propagating, change the density structure of their carrier material. This process gives these signals a direct feedback to the gravity potential, which originates from the density distribution. Their detection thus makes it possible to map the long wavelength and therefore large scale density structure. Generally, there are two types of long period signals: tides with a periods of multiple hours, and normal modes or free oscillations with periods from seconds up to one hour for the Earth. To excite free oscillations in planets a powerful seismic energy source is needed. Although the amount of high-quality long-period data has increased significantly in recent decades due to the occurrence of several very large quakes and high precision satellite measurements, there is still little agreement between different density models.
The numerical description of normal modes started together with the emergence of the computer. Several methods based on different forms of numerical integration have been proposed to solve the physically motivated system of differential equations. To resolve density, accuracy of the solutions is of foremost importance as the magnitude of variations is assumed to be in the same order of the error of commonly employed numerical approximations.
The present work describes our advances in long-period seismology with a strong emphasis on our numerical developments which culminated in the open-source publication of a new spectral element based normal mode code called specnm. This highly accurate numerical forward code enables us to invert the most recent normal mode measurements to obtain new one-dimensional Earth structure models with higher resolution in density than ever before. To improve on former modelling approaches, we create self-consistently built models of the radial anelastic seismic structure of the Earth. For this, we construct a petrologically and thermodynamically consistent mantle structure which we unite together with a laboratory-based visco-elastic attenuation model that connects dissipation from seismic to tidal periods, whereas core properties are computed using equations-of-state. Lastly, we improve the efficiency of the calculation for three-dimensional simulations in time of gravity affected waves by, among other things, using the crucial fact that the gravity potential outside of the planet has a simple 1/r dependence.
Together, these advances may lead to a better understanding of the density variations in planets. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000614268Publication status
publishedExternal links
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Publisher
ETH ZurichSubject
Normal modes; Seismology; Geophysics; Theoretical seismology; Inversion theory; Numerical modelling; Free oscillations; Seismic attenuation; Structure of the Earth; Composition and structure of the mantle; Tides; Time variable gravity; Computational seismology; Probabilistic modellingOrganisational unit
03476 - Giardini, Domenico / Giardini, Domenico
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ETH Bibliography
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