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.
The main goal of the project is to use geodetic data along with normal-mode information from seismology to provide constraints on the mechanical properties of the Earthâ€™s mantle. Three-dimensional solid-Earth tidal amplitudes from a new geodetic solution using a global network of Global Positioning System (GPS) sites will be inverted to yield a three-dimensional model for the elastic parameters and density of the mantle. The project makes use of the modern, dense, global GPS sites for the determination of Earth-tide amplitudes across the spectrum of tidal frequencies to yield site-dependent corrections to the (three-dimensional) Love numbers, the normalized response of the Earth to the tidal forcing of (primarily) the sun and moon. The activities in this project can be grouped under the following project goals: Develop and refine the theory for utlizing geodetic data to infer the density structure of Earth. This involved applying the seismic theory of "normal modes" that are usually used to describe how the Earth resonates following an earthquake to deformations at tidal frequencies (approximately 1 cycles per day (cpd), 2 cpd ...) Develop and refine the approach used to estimate the tidal spectrum from daily GPS data. The tidal spectrum is made of of a number of contributions ("tidal species") grouped closely together in frequency (for example, 1.0000 cpd and 0.9664 cpd). Usually, a very long time series is required to separate closely spaced species, but geodetic data are analyzed in daily batches. Analyze a decade-long timespan of data from a global network of ~100 GPS sites to estimate the tidal spectrum at each site. (Actually, the deviation from a nominal spectrum based on a reference Earth model is estimated.) Using the results of 1-3, estimate a three-dimensional model for the density structure of the Earth's interior. We coined the term "tidal tomography" for the approach for estimating Earth structure using the tides, and has had a broad impact. This new approach has been used by others, for example, to estimate the structure of temperature and elastic properties of the Earth beneath the western U.S. using the NSF-sponsored Plate Boundary Observatory, and to investigate whether tidal tomography can be used to determine the structure of the Moon. The tidal theory also has broad applicability, for example, in determining the response of marine glaciers to ocean tides, which reveals important properties of glacier structure and behavior. The final analysis will be reported at the Fall 2013 Meeting of the American Geophysical Union. The project has been used to provide training and development for two graduate students (Ph.D.) and one postdoctoral research scientist.