Study of Inner Core Motions using Seismic Waveform Doublets

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This project is to study time-dependent features of the inner core, including inner core rotation, based on measurements made on seismic waves derived from waveform doublets. (Waveform doublets are the seismic signals from two earthquakes occurring at different times but in almost exactly the same location, so that their waveforms look very similar at each station recording both events.) The new approach, based on a search for waveform doublets, avoids contamination from unknown lateral heterogeneities in the Earth and allows the research to focus on the interpretation of very precise measurements of the travel-time change between doublets signals that have traversed the inner core.The intellectual merit of this approach is that it promotes recognition of the dynamic nature of the Earth's deepest interior; and that it has implications for the big picture of how our planet differentiated, and the degree of vigor with which convection in the fluid core is maintained. By measuring changes within the inner core on a time scale that can lead to perceptible effects over a decade or less, there are consequences in the study of Earth's overall heat flow and energy balance; geomagnetism; and gravity, as well as seismology. (These are the four geophysical disciplines associated with observable features measurable at the Earth's surface).The work has broader impact because (a) it is a new twist on research that led to hundreds of news stories in the 1990s; (b) it is quite easily understandable by people of all ages who lack scientific training but who are curious about the planet on which we live; and most fundamentally because (c) the inner core, as a Moon-sized object apparently moving inside the Earth, is a fascinating object, and the background to good stories of how science is done.The investigator's methods for studying travel-time change are also a contribution within the general subject of precision seismology, in which even teleseismic signals are measured with attention to travel time changes at the level of ~ 10 milliseconds. The discovery of new waveform doublets in subduction zones is of great interest in the context of earthquake physics, whether the events occur close together in time or are separated by several years.