Inner-core Fine Structure and Rotation from Array Studies

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0001126VidaleThis project proposes to measure the fine-scale structure of the Earth's inner-core and estimate its rotation using newly discovered inner-core-scattered waves (ICS). The PIs recently showed an extended ( 200 s) scattered wavetrain following the inner-core-boundary reflection (PkiKP), which can be explained by short-wavelength (3.5 km) small amplitude ( 3%) stiffness variations distributed across the outer half of the inner-core. These initial results indicate the inner core is one of the more heterogeneous places in our planet. This project would amass a database of array ICS waves from different epicentral ranges, the research will be able to place constraints on the amplitude variation of distinct physical properties. Analysis of ICS waves provides a simple test for inner-core motion. Scattered waves generated by two Novaya Zemlya nuclear tests separated by less than a kilometer occurring in 1971 and 1974 recorded at LASA show small but coherent changes in the ICS waves. Energy scattered in the inner core to the west of the great-circle path is up to 0.1 s earlier in 1974 than 1971. The opposite trend appears for scattering from the east. This pattern suggests an inner-core rotation rate of 0.15 degrees per year. Investigation of additional events is needed to confirm the initial result, test for the temporal stability on the inner-core-rotation rate, and determine the inner-core super-rotation pole.