PROBING THE LITHOSPHERE-ASTHENOSPHERE SYSTEM WITH SCATTERED SEISMIC WAVES

Hua, Junlin

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Year:: 2021
Contributor:: Hua, Junlin (creator); Fischer, Karen (Advisor); Dalton, Colleen (Reader); Tsai, Victor (Reader); Liang, Yan (Reader); Gazel, Esteban (Reader); Brown University. Department of Earth, Environmental, and Planetary Sciences (sponsor)
Genre:: theses
Subject:: Geodynamics; Geophysics; Seismology; Upper Mantle; Plate tectonics
Extent:: xxvi, 321 p.

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Abstract:: The lithosphere-asthenosphere system is one of the most fundamental components of the Earth, as it sustains the existence of plate tectonics and shapes the surface of the planet. This dissertation first studies the behavior of Sp seismic scattered waves originated from the lithosphere-asthenosphere system to develop novel seismic imaging techniques, and then applies them to study the system at local, regional and global scales. In the South Island of New Zealand (Chapter 1), common conversion point stacks of converted waves were used to image the lithosphere-asthenosphere boundary across the Alpine Fault. On the Australian side of this strike-slip plate boundary, the lithosphere-asthenosphere boundary is deeper and wider than on the Pacific side, indicating either partial subduction of the Australian plate or underthrusting of the Pacific plate. However, these two hypotheses could not be distinguished because of the assumption of horizontal discontinuities that underlies common conversion point stacks. Therefore, I studied the sensitivity of Sp scattered waves to discontinuities in detail (Chapter 2) to develop a method for imaging dipping structures. The results show that Sp scattered waves only have limited sensitivity to dipping discontinuities, but SKSp waves show a much better imaging capability for moderately dipping structures. I also developed a new pre-stack migration imaging method. Despite their low sensitivity to dipping discontinuities, Sp scattered waves are very sensitive to horizontal structures. Hence, based on the near-horizontal sensitivity kernels of Sp receiver functions, I derived a new common-conversion point stacking method (Chapter 3), and applied it to the Anatolian region. The resulting stacks reveal a rarely observed positive seismic velocity gradient with depth beneath most of the region. Combining the stacked receiver functions with local erupted basalt samples (Chapter 4), the observed velocity gradient is found to mark the bottom of a partially molten mantle layer due to elevated temperature. The heat source of the Anatolian hot mantle was then traced back to the East African Rift system located more than 2000 km away with the help of geochemical data. We found the temperature reduction along this upper mantle transport path is minimal, based on petrological samples. This small temperature change can be explained by high rates of asthenospheric flow in response to plume-induced pressure-driven flow. Finally, positive velocity gradients similar to those in Anatolia were analyzed globally (Chapter 5), and were found to be present at ~150 km depth beneath ~35% of the Earth’s area. Similar to Anatolia, this semi-global discontinuity corresponds to the bottom of a partially molten layer, and could act as a viscosity boundary separating a shallower and weaker partially-molten asthenosphere layer from deeper and stronger ambient asthenosphere.
Notes:: Thesis (Ph. D.)--Brown University, 2021

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Citation

Hua, Junlin, "PROBING THE LITHOSPHERE-ASTHENOSPHERE SYSTEM WITH SCATTERED SEISMIC WAVES" (2021). Earth, Environmental and Planetary Sciences Theses and Dissertations. Brown Digital Repository. Brown University Library. https://repository.library.brown.edu/studio/item/bdr:n4r8bv36/

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Earth, Environmental and Planetary Sciences Theses and Dissertations

Theses and Dissertations for the Earth, Environmental and Planetary Sciences department.
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