Assistant Professor Christopher Harig has been awarded a three-year, $460,000 grant from NASA’s Earth Surface and Interior program for his project, “Determining Earth Structure from the Local Dynamic Geoid." The main goal of the project is to study regional differences in the strength of the upper mantle by linking regional mantle dynamics to the gravity field. Dr. Harig’s research group at the UA studies ice mass losses from ice sheets like Antarctica. Antarctica is still uplifting in response to the most recent deglaciation (~20 kya). Understanding the strength of the mantle and having a better model of this deformation will let us better estimate ice losses happening today.
Beginning in the late 1980s and continuing since, studies have related the density structure of the mantle, estimated by seismic tomography, and its flow to the gravity field observed at the surface to infer the rheology of the mantle. This work was accomplished mainly by two methods. In the first method, one-dimensional ’geoid kernels’ were developed which are semi-analytic Green’s functions relating density anomalies in the mantle to gravity observations at the surface. In the second method, fully numerical (e.g. finite-element) 3D calculations also solve the forward problem, but at greater computational cost. If we wish to consider lateral variations in mantle viscosity we have, until now, been limited to using fully numerical calculations. In this project, we update the classical semi-analytic method to study lateral variations in mantle rheology by altering the observables used (i.e. the gravity field) so that the relation between density anomalies and gravity remains 1D in a LOCAL region.
By representing the gravity field in an alternate basis set to Spherical Harmonics, Slepian functions, we localize NASA’s data to specific regions of the globe and focus on the mantle rheology of the western United States, comparing this region with the rheology that can be determined globally. We will use observations of the static gravity field which have significantly improved since the launch of the GRACE (Gravity Recovery and Climate Experiment) satellite mission in 2002. Also in this region recent improvements in seismic models will improve the resulting resolution compared to using global seismic models.