Structure of the earth’s topmost inner core from seismic waveform inversion

Seismology;Inner core;Anisotropy;Waveform inversion

Jin, Jing

关键词: Seismology;    Inner core;    Anisotropy;    Waveform inversion;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/98195/JIN-DISSERTATION-2017.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

The Earth’s solid inner core was formed from crystallization of liquid iron in the fluid outer core as the Earth slowly cools. The understanding of the structure of the inner core, combined with high pressure and temperature experiments at core condition, and dynamic simulation, will enable us to better understand the most remote part of the Earth. Numerous seismological studies have shown great complexities of the Earth’s inner-core structure. The lateral variation of the inner-core structure is just as pronounced with hemispherical variation in isotropic velocity of the topmost inner core, in attenuation, and in seismic anisotropy. This variation is very important in studying the outer-core convection and inner-core solidification process, which is the driving mechanism of the geodynamo and the Earth’s magnetic field. However, some key features of the inner core have not been well understood, such as the relation between velocity and attenuation structure, and the global pattern of the lateral variation. In this dissertation, I put my focus on studying the velocity and attenuation structure of the topmost inner core. To address these issues, my dissertation involves two main parts:First, to map the structure of the topmost inner core, I have developed an automatic waveform inversion technique based on Neighborhood Algorithm. This method takes advantage of the triplicated PKP waveforms at epicentral distances from 130 to 142 degrees and allows us to model P-wave velocity and attenuation structure of the inner core simultaneously. It is proven to be able to resolve the structure accurate and efficient in the synthetic tests.Second, I used this waveform inversion technique to study the global patterns of the lateral variation of the topmost inner core. I conducted a systematic search of high-quality waveform data with the goal of good global data coverage and eventually selected 31 global seismic events in 1990 – 2015 with more than 2,000 seismograms. The inversion result shows strong degree 1 hemispherical velocity and attenuation structure, which agrees with previous studies. We also observed strong anisotropy of P-wave velocity at the topmost inner core in some region, which has not been discovered before. Moreover, we observed degree 2 and degree 3 structures at the topmost inner core. Both the velocity and attenuation structures correlate well with the long- wavelength features of the lowermost mantle, suggesting a strong influence of the mantle on the core.