Zheng, Yingcai2020-01-04May 20192019-05May 2019https://hdl.handle.net/10657/5773Detailed geomorphology of the 410-km phase-transition discontinuity offers important information about the thermal state and dynamics of the Earth interior. We generated a high-resolution image of the 410-km discontinuity around the Tonga subducting slab using a three-dimensional pre-stack elastic Kirchhoff migration of signals from deep-focus earthquakes. This provides a great opportunity for us to examine slab mantle interactions. We found from both P and S images, the depth of the 410-km discontinuity inside the subducting slab is observed ~30 km shallower relative to its depth outside of the slab. The shallower depth of the 410-km discontinuity and strong reflectivity of the p410P and s410S within the slab provide evidence against pervasive metastability of the olivine-wadsleyite phase change as a mechanism of deep-focus earthquakes. Further to the west, under the present-day Fiji Islands, the 410-km discontinuity is depressed by ~20 km which supports the hypothesis of mantle plume migration from Samoa chain islands to Fiji. Assuming equilibrium phase change, we infer a temperature difference between the cold slab and hot plume of ~530-820 K, depending on uncertainties in mineralogy and the derived Clausius-Clapeyron slope. Such a large temperature gradient may drive complex slab mantle-interaction patterns. Possible matches of velocity and density across the 410-km discontinuity have been found through amplitude information. Combined with velocity and impedance contrasts from previous experimental studies, we think a pyrolytic transition zone with 1.79% to 2.34% water content fits our observation.application/pdfengThe author of this work is the copyright owner. UH Libraries and the Texas Digital Library have their permission to store and provide access to this work. Further transmission, reproduction, or presentation of this work is prohibited except with permission of the author(s).410-km DiscontinuityHigh-resolution ImagingSlab-Plume InteractionWater Content in Transition Zone2020-01-04Thesisborn digital