Investigating the Internal Structure of Earth and Mars with Seismic Body Waves
Schmerr, Nicholas C.
MetadataShow full item record
Seismic waves propagating through the interior of planetary bodies arepowerful imaging tools for revealing a high-resolution picture of their internal structures. Owing to the abundant seismic data on Earth, seismology has provided robust constraints on Earth’s 1-D and 3-D internal structures. Deployments of seismometers on other terrestrial planets via spacecraft missions has opened the door to explore the interior of these planets through planetary seismology. My dissertation seeks to understand the mantle structures and dynamics of Earth and Mars using a joint approach of seismic data analysis and synthetic waveform modeling. I utilized a body wave approach, SS precursors, to investigate the topography and seismic anisotropy structures of Earth’s mantle transition zone (MTZ). On Mars, I investigated the signatures of a seismic discontinuity associated with the olivine-to-wadsleyite phase transition in martian mantle using seismic data recorded by NASA’s InSight Mission. Global topography of MTZ discontinuities is characterized by regional thinning beneath hot spots and thickening beneath subduction zones, indicating mantle temperature plays a crucial role in the topography of MTZ discontinuities. I demonstrated with 3-D synthetic modeling that SS precursors can detect at least 3% azimuthal anisotropy in the MTZ as well as distinguish anisotropy from the shallow and deep upper mantle. I observed azimuthal anisotropy in the MTZ beneath subduction zones with SS precursors and the fast directions are predominantly trench-perpendicular, which is attributed to the lattice preferred orientation of wadsleyite. This is interpreted as the 3-D toroidal flow caused by trench migration. On Mars, I investigated the detectability of the MTZ, and found that that triplicated waves are the most suitable phases for sensing the olivine phase changes. I combined a polarization filter and vespagram techniques to identify body waves in InSight data. I discovered the existence of multiple reflected waves in the near-field, and evidence for triplicated waves in the far-field after aligning Marsquakes on P- and S-arrivals. Preliminary depth estimate of olivine-to-wadsleyite phase transition from the triplications indicates a cold or hydrated martian mantle. A new seismology-based picture of the martian interior is emerging from my work on the InSight data.