Seismological Structure of the Earth's Lowermost Outer Core (F Layer) Beneath the East-Central Pacific

Abstract

During the inner core solidification, excess light elements are released into the outer core, which causes outer-core convection. To understand this process, it is important to determine the velocity structure of the bottom outer core (F layer). In previous studies, we developed an effective method that combines two independent observations to determine the structure. The first is the frequency dispersion of the traveltime of the wave creeping along the inner core boundary (PKPbc and PKPc-diff) relative to the wave passing through the inner core (PKIKP). The second is the traveltime difference between the reflected wave at the boundary (PKiKP) and a wave passing nearby (PKPbc). We applied this method to two laterally separated regions and observed differences in the velocity structure. In this study, we applied the same technique to a third region immediately south of one of the previously analyzed regions. Our analyses show that the observations of the study area can be explained by the velocity model for the northward neighboring area. Our analyses also confirm that no factors other than the F-layer structure can significantly affect the observed dispersion. Additionally, the distance decay rates of the PKPc-diff amplitudes were consistent with the predictions made using the same model, further confirming the stability and reliability of our method. We also show the results of converting the lateral velocity difference of the F layer between the two regions obtained in a previous study into the relative abundance of light elements, such as sulfur, silica, oxygen, carbon, and hydrogen.