Seismic imaging of deep mantle plumes and inner-outer core boundary heterogeneity using core waves
Creators: name: Nelson, Peter
Publisher: name: The University of Texas at Austin
Year: 2020
Subjects: Mantle plumes
Yellowstone
Core waves
ICB
Outer core heterogeneity
F-layer

description: The large contrast of physical properties across the core-mantle boundary (CMB) results in complicated seismic raypaths that can be used to study the deep earth. In this dissertation, I use core waves to address two controversial topics in earth science: the existence of deep thermal mantle plumes and lateral heterogeneity at the base of the outer core. First, I test the plume hypothesis by using SKS and SKKS travel times to construct a shear wave tomography model for the mantle beneath the Yellowstone Hotspot in western North America. The model is optimized to find short wavelength, subvertical structures in the lower mantle. I chose the Yellowstone Hotspot over other more prominent hotspots because it is the only hotspot with a purported deep origin that is located in a continental interior with seismic networks dense enough to image a thin mantle plume in the lower mantle. The shear wave tomography model for the deep mantle beneath the western United States that I present is constructed using the travel times of SKS and SKKS recorded by the dense USArray seismic network. SKS and SKKS are more sensitive to lower mantle plumes than other phases traditionally used for mantle tomography because of their subvertical raypaths in the lower mantle. The model has a single narrow, cylindrically shaped slow anomaly, approximately 350 km in diameter that I interpret as a whole-mantle plume. The anomaly is tilted to the northeast and extends from the CMB to the surficial position of the Yellowstone Hotspot. The structure gradually decreases in strength from the deepest mantle towards the surface. If it is a purely thermal anomaly the peak velocity anomaly at the base implies an initial excess temperature between 650 to 850 °C. These results strongly support a deep origin for the Yellowstone hotspot and provide evidence for the existence of thin thermal mantle plumes that are currently beyond the resolution of global tomography models. I next use PKP to investigate the seismic structure around the inner-outer core boundary (ICB). A zone with reduced P-velocity gradient at the bottom of the outer core, known as the F-layer, has been reported by seismological studies and incorporated into some global 1-D Earth models. However, not all studies have found such a feature, particularly beneath the eastern hemisphere. Most seismic studies have used differential travel time measurements between single pairs of PKP waves to study the ICB, but such studies result in tradeoffs between outer and inner core structure leading to ambiguous conclusions about the F-layer. Here I minimize that tradeoff by simultaneously waveform modelling all branches of PKP for two different datasets, one sampling the eastern hemisphere and another sampling the western hemisphere. The datasets clearly show all PKP waves and were made by stacking seismograms from events with high signal to noise ratios recorded at dense networks in China and Japan. I use a stochastic waveform inversion method to determine a 1D seismic model for each hemisphere that fits the data. For the eastern hemisphere, my best fitting model has a thin 60-100 km thick high velocity lid at the top of the inner core and no anomalous gradient in the F-layer. The origin of the high velocity lid is unclear, but likely causes are abrupt change in anisotropy, an enrichment in light elements, or a thermal boundary layer if the inner core has a low thermal conductivity. Further tests also show that a reduced velocity gradient in the F-layer cannot simultaneously fit all the PKP arrivals. In contrast, my best fitting model for the western hemisphere has a reduced velocity gradient in the F-layer and a thick low velocity zone beneath the ICB that is most likely caused by seismic anisotropy. These two results suggest that the F-layer is laterally The large contrast of physical properties across the core-mantle boundary (CMB) results in complicated seismic raypaths that can be used to study the deep earth. In this dissertation, I use core waves to address two controversial topics in earth science: the existence of deep thermal mantle plumes and lateral heterogeneity at the base of the outer core. First, I test the plume hypothesis by using SKS and SKKS travel times to construct a shear wave tomography model for the mantle beneath the Yellowstone Hotspot in western North America. The model is optimized to find short wavelength, subvertical structures in the lower mantle. I chose the Yellowstone Hotspot over other more prominent hotspots because it is the only hotspot with a purported deep origin that is located in a continental interior with seismic networks dense enough to image a thin mantle plume in the lower mantle. The shear wave tomography model for the deep mantle beneath the western United States that I present is constructed using the travel times of SKS and SKKS recorded by the dense USArray seismic network. SKS and SKKS are more sensitive to lower mantle plumes than other phases traditionally used for mantle tomography because of their subvertical raypaths in the lower mantle. The model has a single narrow, cylindrically shaped slow anomaly, approximately 350 km in diameter that I interpret as a whole-mantle plume. The anomaly is tilted to the northeast and extends from the CMB to the surficial position of the Yellowstone Hotspot. The structure gradually decreases in strength from the deepest mantle towards the surface. If it is a purely thermal anomaly the peak velocity anomaly at the base implies an initial excess temperature between 650 to 850 °C. These results strongly support a deep origin for the Yellowstone hotspot and provide evidence for the existence of thin thermal mantle plumes that are currently beyond the resolution of global tomography models. I next use PKP to investigate the seismic structure around the inner-outer core boundary (ICB). A zone with reduced P-velocity gradient at the bottom of the outer core, known as the F-layer, has been reported by seismological studies and incorporated into some global 1-D Earth models. However, not all studies have found such a feature, particularly beneath the eastern hemisphere. Most seismic studies have used differential travel time measurements between single pairs of PKP waves to study the ICB, but such studies result in tradeoffs between outer and inner core structure leading to ambiguous conclusions about the F-layer. Here I minimize that tradeoff by simultaneously waveform modelling all branches of PKP for two different datasets, one sampling the eastern hemisphere and another sampling the western hemisphere. The datasets clearly show all PKP waves and were made by stacking seismograms from events with high signal to noise ratios recorded at dense networks in China and Japan. I use a stochastic waveform inversion method to determine a 1D seismic model for each hemisphere that fits the data. For the eastern hemisphere, my best fitting model has a thin 60-100 km thick high velocity lid at the top of the inner core and no anomalous gradient in the F-layer. The origin of the high velocity lid is unclear, but likely causes are abrupt change in anisotropy, an enrichment in light elements, or a thermal boundary layer if the inner core has a low thermal conductivity. Further tests also show that a reduced velocity gradient in the F-layer cannot simultaneously fit all the PKP arrivals. In contrast, my best fitting model for the western hemisphere has a reduced velocity gradient in the F-layer and a thick low velocity zone beneath the ICB that is most likely caused by seismic anisotropy. These two results suggest that the F-layer is laterally heterogeneous which has important implications for core dynamics such as the possibility of an increase in viscosity with depth in the outer core.