EAR-PF: Shear Wave Splitting based on 3D Seismic Wave Simulations: Forward to Inverse Modeling of Upper Mantle and D" Anisotropy

Creasy, Neala Marie, New Haven CT

Investigators

Abstract

Dr. Neala Creasy has been granted an NSF EAR postdoctoral fellowship to carry out research and educational plans at Colorado School of Mines (CSM). She will investigate how the Earth's mantle deforms under high pressures and temperatures by making use of the seismic waves produced by earthquakes. Interpreting these seismic waves and how they directly relate to these deformation and mineralogical processes within the mantle is difficult, in part due to necessary assumptions and fundamental limitations inherent to mineral physics experiments at high pressures and temperatures. She will calculate synthetic waveforms for a 3D, realistic Earth to explore how seismic observations relate to mantle deformation. While some prior research has explored the viability of some of the assumptions used to simplify these observations, there are many additional aspects that need to be fully explored to fully understand the complexity of the Earth. Understanding how the Earth deforms via mantle convection is important because this deformation controls the surface expression of plate tectonics, in the form of volcanic activity and earthquakes. This research will help clarify the general understanding of how to use seismic waves to their full potential in understanding current processes in Earth's mantle. Her educational plan involves acting as a research mentor for graduate and undergraduate students at CSM, creating educational material (e.g., infographics, teaching materials) and a Virtual Reality (VR) setup to excite young scientists to pursue basic science, and continued outreach efforts in the local community through organizations including IRIS (Incorporated Research Institutions of Seismology undergraduate internship program) and the Denver Museum of Science. Constraining the pattern and properties of seismic anisotropy in the Earth can help reveal relationships between mineral physics, mantle convection, and seismology. Sources of anisotropy in the lithosphere as frozen-in anisotropy, transition zone, and D" complicate shear wave splitting measurements, resulting in shear wave splitting that can differ from plate motion. If we better understand seismic anisotropy sourced in the lithosphere, we could also better constrain D" anisotropy, which requires correcting for the upper mantle to some extent. Ray theory is commonly used and is appropriate within certain limits, but not all implications have been explored. Ray theory is an infinite frequency approximation and its validity depends on the period of waves, the scale of heterogeneities, the length of its propagation path, and the superposition of multiple arrivals, making interpreting seismic anisotropy observations more difficult. Numerical methods and advances in high-performance computing offer new opportunities to take the full physics of wave propagation into account using realistic 3D Earth structures while conducting seismological observational studies. In this work, Dr. Creasy will explore the assumptions made in shear wave splitting as well as tease out discrepancies between different models and observations of anisotropy within the Earth, by conducting numerical simulations via 3D global wave propagation solver SPECFEM3D_GLOBE. This work will help improve the understanding of how seismic anisotropy observations are related to models of deformation in the Earth's mantle and the sources of anisotropy in the various regions of the crust, upper mantle, and D". These insights will help illuminate discrepancies between different seismic anisotropy observational techniques on regional (e.g. North America and Australia) and global scales. This work will also assist the development of using shear wave splitting in global full waveform inversion addressing appropriate parametrization to describe body-wave anisotropy in the mantle during the inversion process. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.