Earth's Internal Processes and Structure Based on Novel Seismological Approaches

Abstract

Earth is an active planet, generating various indigenous sources and, at the same time, responding to them as a medium. This thesis covers a wide range of topics in seismology, including earthquake rupture, near-surface seismic structure, and the variations within the mantle transition zone. Despite these disparate subjects, there is a common theme: the development of novel approaches to expand our ability to study the Earth's internal processes. I have developed a method to examine three-dimensional complexity of the spatiotemporal evolution of deep rupture processes, while the conventional approaches assume the problem to be two dimensions. This approach uses the Doppler effect, i.e., stretching and shrinking of waveforms, to investigate if the rupture propagates linearly and uni-/bi-directionally, or in a more complex pattern. The method has been applied to two deep (~600 km) earthquakes in the Kuril subduction zone and revealed that they have very different three-dimensional geometry despite occurring within the same slab. The complex rupture pattern also points to the existence of a complicated stress field, contrary to the relatively uniform stress field typically assumed or argued for shallow earthquakes. In addition, I have introduced another technique for imaging small-scale (<100 km) variations in 410- and 660-km discontinuities. Based on the technique, I find broad or missing 410-discontinuities that are difficult to explain only by high water content, and the variable sharpness suggests patchy melt pockets with high temperature and high iron content. Moreover, "double 660-km discontinuities" separated by only 10 to 20 km are observed, each corresponding to Ilmenite to Bridgmanite and post-spinel transitions. The former exhibits steep Clapeyron slope that can potentially inhibit mantle convection in subduction zones. In my third project, I have developed a new approach to estimate the seismic wave speed near the surface (~100 m) by extending the Snell's law to show that wave speed beneath a seismometer controls the recorded orientations of body-wave arrivals. It has been applied to Japan, generating a map of near-surface wave speeds that is found to correlate with topography, volcanoes, and geology. This technique opens new paths for earthquake hazard assessment that is reliable, non-invasive, and both computationally and financially inexpensive, and that can be applied in any part of the world. The technique is also extended to constrain variations of wave speeds with depth based on observations of frequency-dependent polarization, which is applied to broadband stations in Japan and United States. Moreover, systematic inspection of the body-wave orientations has effectively identified instrumental issues that have not been reported previously. The inspection procedure is useful for operating and maintaining seismic instruments, especially for difficult-to-access ocean-bottom or borehole sensors.