Investigating the Internal Structure of Earth and Mars with Seismic Body Waves
Investigating the Internal Structure of Earth and Mars with Seismic Body Waves
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Date
2020
Authors
Huang, Quancheng
Advisor
Schmerr, Nicholas C.
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Abstract
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.