Determining the structure of the Earth's mantle is a fundamental problem of geophysical research. This information, especially the variation in density, provides important insight into the composition and dynamics of the Earth's deep interior. Seismological data have been used to construct models of the mantle, but the data conventionally used for such purpose (body and surface waves) are not sensitive to density variation. A global density model is typically obtained by making a strong assumption that seismic wave speed variations are linearly related to density variation. This assumption loosely translates to three-dimensional variations arising from a purely thermal origin. Results from recent studies, however, suggest that the density heterogeneity within the mantle is poorly correlated with shear-wave speed anomalies, especially in the lower mantle. In this project, we combine multidisciplinary data types to better-constrain properties of the Earth's mantle with a focus on density.
The modern, dense, global GPS sites will be used for the determination of Earth-tide amplitudes across the spectrum of tidal frequencies to yield site-dependent corrections to the (three-dimensional) Love numbers. Previous studies using other ground-based space geodetic systems indicate that the GPS network has great sensitivity to the Love numbers, in part due to the unique temporal spectrum of the tides. These Love numbers are usually calculated using a spherically symmetric Earth model.
The proposed research would explore an approach for combining global geodetic and seismic data to yield a new model for the mechanical structure of the Earth. Seismological data (normal mode splitting information) from previous studies, three-dimensional solid-Earth tidal amplitudes from a new geodetic solution using a global network of Global Positioning System (GPS) sites, and the Earth's gravity field from satellite geodesy will be jointly inverted to obtain a three-dimensional model for the elastic parameters and density of the mantle. Each of these data types has different sensitivities and inherent resolution, and the combination will lead to a model with superior accuracy and resolution relative to the individual techniques.
Determining the mechanical structure of the Earth's interior is a fundamental problem of geophysical research. The structure provides important insight into the chemical composition and dynamics of the Earth's deep interior, as well as our understanding of how our planet responds to internal and external forces. Using waves generated by earthquakes allows us to investigate the structure deep down, but the seismological data conventionally used for mantle imaging are not sensitive to variations in density. A global density model is typically obtained by making some strong assumptions. Resultsfrom recent studies, however, consistently suggest that the density variation within the mantle is poorly modeled using such assumptions. This project investigated the plausibility of using deformation of the Earth from gravitational forcing from the Sun and Moon (solid tides rather than ocean tides) to better understand the mantle structure, in particular, the density variations. Numerical simulations suggest that the associated deformation is small, but is likely observable using a global network of GPS stations. Motivated by this finding, we have started to explore whether this signal can be theoretically formulated by modifying existing analytical techniques in seismology. We have also looked to improve seismological constraints by careful analysis of the data from the 2011 Tohoku, Japan earthquake.
